LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


GSFT    OF 


Class 


r 


ELEMENTARY  PHYSICS 


ELEMEISTTABY 
PHYSIOS 


BY 

FRANK   WILLIAM    MILLER,   A.M. 

AND 

AUGUST   FREDERIC   FOERSTE,   PH.D. 

INSTRUCTORS    IN    PHYSICS,    STEELE    HIGH    SCHOOL 
DAYTON,    O. 


NEW   YORK 

CHARLES    SCRIBNER'S    SONS 
1903 


N\S' 


COPTBIGHT,   1903S   BY 

FRANK  WILLIAM  MILLER 

AND 

AUGUST  FREDERIC  FOERSTB 


,        ..,...:•- 
•,.•  ::,•:';  : 


TROW  DIRECTORY 

PRINTING  AND  BOOKBINDING  COMPANY 
NEW  YORK 


CA" 


PREFACE 

THE  purpose  of  this  book  is  to  make  the  pupil  ac- 
quainted with  the  more  elementary  facts  of  physics 
and  physical  chemistry,  to  give  some  idea  of  methods 
of  experimentation,  to  illustrate  the  drawing-  of  conclu- 
sions from  experiments  and  observations,  and  to  show  that 
theories  are  merely  attempts  to  explain,  by  means  of  cer- 
tain suppositions,  various  phenomena  whose  existence  is 
unquestioned  but  whose  nature  can  not  be  more  satisfac- 
torily explained  by  other  means. 

In  order  to  secure  these  results  within  the  time  usually 
devoted  to  physics  in  a  high-school  course,  it  is  necessary 
to  limit  the  number  of  phenomena  selected  for  study.  In 
this  selection  we  have  been  influenced  not  only  by  the 
desire  to  choose  those  phenomena  most  worthy  of  study, 
but  also  by  the  necessity  of  confining  the  choice  to  those 
phenomena  which  admit  of  a  logical  treatment  within 
the  limits  of  this  book.  It  is  considered  inexpedient 
to  attempt  to  include  all  matters  of  popular  interest.  If 
the  material  here  selected  does  not  fully  occupy  all  the 
time  of  the  pupil,  the  teacher  may  with  profit  add  a  dis- 
cussion of  various  other  subjects  of  direct  interest  to  his 
immediate  community.  The  arrangement  of  this  material 
in  chapters  is  due  to  the  following  considerations. 

Many  of  the  phenomena  shown  by  gases  and  liquids 


vi  PREFACE 

may  be  illustrated  by  means  of  very  simple  apparatus. 
Their  explanation  is  usually  simple  and  does  not  involve 
the  use  of  theories.  For  this  reason  their  study  is  espe- 
cially adapted  to  serve  as  an  introduction  to  methods  of 
experimentation  and  direct  reasoning. 

Next  we  endeavor  to  show  that  various  phenomena, 
otherwise  inexplainable,  can  be  understood  if  a  certain 
theory  be  accepted.  A  theory,  in  order  to  be  of  any  real 
service,  must  rest  upon  a  very  good  basis  of  judiciously 
observed  facts.  For  this  reason  we  believe  that,  before 
a  theory  is  advanced,  there  should  be  presented  a  series 
of  phenomena  which  can  be  explained  only  by  means  of 
that  theory. 

For  instance,  in  the  second  chapter,  in  order  to  make 
clear  the  need  of  the  Molecular  theory,  we  discuss  cer- 
tain of  the  more  elementary  phenomena  of  heat,  diffu- 
sion, osmosis,  evaporation,  solution,  and  crystallization, 
and  show  that  a  comprehension  of  these  phenomena  is 
materially  assisted  by  the  Molecular  theory.  This  is 
followed  in  the  third  chapter  by  a  more  extended  ac- 
count of  those  phenomena  of  heat  which  can  be  ex- 
plained by  the  use  of  the  Molecular  theory  without  the 
addition  of  the  Ether  theory. 

The  fourth  chapter  includes  the  study  of  the  Atomic 
theory,  the  foundation  of  chemistry.  This  theory  de- 
mands greater  powers  of  imagination.  It  is  not  possible 
in  support  of  it  to  produce  so  many  simple  experiments, 
easily  performed  and  easily  understood,  which  show  the 
connection  between  the  phenomena  considered  and  the 
theory  by  means  of  which  they  can  be  explained. 


PKEFACE  vii 

The  fifth  chapter  takes  up  enough  of  the  subject  of 
sound  to  bring  out  the  idea  of  vibrations  and  of  waves. 

This  idea  is  utilized  in  the  sixth  chapter,  which  is  espe- 
cially devoted  to  a  consideration  of  the  Ether  theory. 
This  is  one  of  the  most  abstruse  theories  of  science.  Spe- 
cial emphasis  should  be  placed,  therefore,  upon  the  rea- 
sonableness of  this  theory  as  a  true  explanation  of  the 
facts.  The  phenomena  studied  are  those  of  radiant 
energy.  In  the  treatment  of  this  subject  as  little  distinc- 
tion as  possible  is  made  between  those  vibrations  of  the 
ether  which  produce  both  heat  and  light  and  those  which 
produce  only  heat. 

The  Molecular,  Atomic,  and  Ether  theories  are  attempts 
to  get  at  the  facts  underlying  the  phenomena  considered. 
The  result  is  that  most  physicists  believe  in  the  actual 
existence  of  molecules,  atoms,  and  the  ether,  although 
they  may  not  have  clear  conceptions  of  the  form  in  which 
they  exist. 

Lines  of  Force  (seventh  chapter),  as  used  to  explain 
the  phenomena  of  Magnetism,  are  excellent  examples  of 
what  may  be  called  merely  useful  conventions.  No  phys- 
icist believes  in  the  actual  existence  of  these  Lines.  To 
him  Lines  of  Force  represent  merely  the  directions  in 
which  the  magnetic  forces  act.  Nevertheless,  these  con- 
ventions have  proved  very  useful  in  the  correlation  of 
widely  different  phenomena  of  magnetism  and  electro- 
magnetism. 

A  very  useful  theory  for  which  there  is  almost  no 
foundation  is  the  Theory  of  Gravitation  (eighth  chapter). 
While  it  has  been  known  in  all  ages  that  an  apple  will 


viii  PREFACE 

fall  to  the  ground,  and  that  it  falls  with  a  certain  increase 
of  velocity,  it  is  by  no  means  certain  that  the  earth 
actually  attracts  the  apple. 

We  hope  that  the  book  will  serve  as  an  introduction  to 
present  methods  of  scientific  thought  and  research.  It  is 
descriptive  in  character.  The  experiments  discussed 
should  all  be  performed.  In  most  cases  it  will  be  suffi- 
cient if  the  experiments  are  performed  by  the  teacher 
before  the  class.  To  these  experiments  may  be  added 
many  others  which  should  be  performed  by  the  pupil. 
In  order  to  be  of  educative  value,  most  of  the  experi- 
ments performed  should  be  chiefly  quantitative,  although 
readily  within  the  mental  grasp  of  the  pupil.  Since 
the  nature  of  these  additional  experiments  must  depend 
very  largely  upon  the  equipment  at  hand  in  each  school, 
their  selection  may  be  left  to  the  individual  teacher.  The 
list  of  laboratory  experiments  accepted  for  entrance  by 
the  association  of  colleges  will  be  found  very  suggestive. 

The  first  four  chapters  may  be  taught  with  profit  in  the 
first  half  of  the  first  year  in  high-school.  They  include 
most  of  the  facts  in  physics  which  teachers  in  botany, 
physiology,  zoology,  physical  geography,  and  chemistry 
like  to  have  their  pupils  know  before  they  enter  upon 
these  special  subjects.  Teachers  in  these  various  sciences 
find  the  time  allotted  to  their  subject  too  brief  to  enter 
upon  a  full  discussion  of  the  physical  phenomena  in- 
volved, and  also  are  confronted  by  the  necessity  of  tak- 
ing up  physical  phenomena  in  the  order  in  which  they 
occur  in  the  text-books  devoted  to  their  own  work.  As 
a  result,  their  treatment  of  the  physical  phenomena  is 


PKEFACE  ix 

often  very  inadequate  and  even  illogical,  and  in  conse- 
quence the  pupil  does  not  get  clear  conceptions  of  the 
points  in  question.  It  is,  therefore,  better  to  present 
the  physical  phenomena  which  are  of  use  in  explaining 
the  facts  of  other  sciences  first  at  greater  length,  in  their 
proper  general  relationship  with  other  physical  phenom- 
ena. In  this  manner  their  special  significance  in  ex- 
plaining the  various  phenomena  studied  in  other  sciences 
will  gain  in  clearness  and  force. 

In  the  preparation  of  this  book  we  have  utilized  every 
available  source.  We  wish  especially  to  express  our  in- 
debtedness to  Miss  Elizabeth  G.  Evans  for  suggestions 
in  connection  with  the  text  and  for  assistance  in  reading 
the  proof-sheets. 

STEELE  HIGH  SCHOOL,  DAYTON,  OHIO, 
February,   1903. 


CONTENTS 

CHAPTER  PAGE 

I.  GASES  AND  LIQUIDS 1 

II.  MOLECULAR  PHENOMENA 76 

III.  HEAT 100 

IV.  CHEMISTRY,  ATOMS 140 

V.  SOUND 214 

VI.     RADIANT  ENERGY— HEAT  AND  LIGHT     .        .        .    235 

VII.     MAGNETISM  AND  ELECTRICITY 288 

VIII.     MECHANICS         .        .        .        .        .        .        .        .365 

APPENDIX    .  .399 

INDEX  .  ....  403 


ELEMENTARY  PHYSICS 


FIG.  l. 


CHAPTEK  I 

GASES  AND  LIQUIDS 

1.  Air  Fills  Many  Spaces  Apparently  Empty. — Insert  a 
lamp  chimney  vertically  half  way  down  into  water  (Fig-.  1). 
How  does  the  height  of  the  water  within 
the  chimney  compare  with  the  height  of 
the  water  on  the  outside  ? 

Hold  in  your  hand  an  empty  tumbler  ; 
that  is,  a  tumbler  which  does  not  contain 
anything  that  can  be  seen.  Is  it  really 
empty,  or  is  it  full  of  some  invisible  ma- 
terial ?  To  determine  this,  place  the  tum- 
bler, mouth  downward,  half  way  under  the  surface  of  the 
water  in  a  glass  jar  (Fig.  2).  Something  in  the  tumbler 
prevents  the  water  from  rising  as  high  within  the  tumbler 
as  the  level  of  the  water  in  the  jar.  This  is  the  air  which 
is  present  in  the  tumbler.  The  water  presses  up  against 
the  air  and  slightly  compresses  it.  At 
the  same  time  the  condensed  air  presses 
down  and  prevents  the  intruding  water 
from  rising  within  the  tumbler  more 
"~"  than  a  short  distance  above  the  mouth. 

In  the  case  of  the  lamp  chimney, 
neither  end  is  closed,  hence,  as  the  chimney  sinks  into 
the  water,  the  air  is  pushed  out  at  the  top.  The  water 

1 


FIG.  2. 


c.;  s« ...  I::ELEME^TARY  PHYSICS 

therefore  rises  in  the  chimney  to  the  level  of  the  water 
in  the  jar  outside  of  the  chimney. 

2.  Gases  Have  Weight. — Beneath  one  of  the  pans  of  a 
delicate  balance  attach  a  hollow  brass  globe  supplied 
with  a  stop-cock,  by  means  of  which  the  passage  leading 
to  the  interior  of  the  globe  can  be  either  opened  or 
closed.  Upon  the  other  pan  place  a  sufficient  number  of 
weights  or  of  shot  to  make  the  beam  of  the  balance  per- 
fectly horizontal.  In  other  words,  counterpoise  the  globe. 

Then,  without  changing 
the  weights,  remove  the 
brass  globe,  pump  out  the 
air,  close  the  stop-cock, 
and  return  it  to  its  former 
position  beneath  the  pan. 
The  side  of  the  balance 
to  which  the  globe  is  at- 
tached now  rises,  showing 
that  there  has  been  a  loss 
in  weight  on  this  side  of 
the  balance  (Fig.  3).  Since 
the  only  material  removed 
from  this  side  of  the  bal- 
ance is  the  air  which  was 

pumped  out  of  the  globe,  the  loss  in  the  weight  of  the 
globe  can  be  due  only  to  the  removal  of  the  air  within  it. 
Therefore  air  has  weight. 

If  the  globe  had  been  filled  with  hydrogen  or  carbon 
dioxide,  instead  of  air,  it  would  have  been  found  that 
these  gases  also  have  weight,  but  that  hydrogen  is  lighter 
and  carbon  dioxide  is  heavier  than  air. 

3  Additional  Experiments  Proving  that  Gases  have 
Weight, — That  air,  hydrogen,  and  carbon  dioxide  differ 


FIG.  3. 


GASES   AND   LIQUIDS 


3 


in  weight  can  be  shown  also  by  a  modification  of  the  pre- 
ceding- experiment.  Make  an  open  paper  box,  about  four 
inches  square  and  four  inches  deep,  and  by  means  of 
strings  attach  it,  open  end  upward,  beneath  one  of  the 
pans  of  the  balance.  A  paper  oyster-bucket  will  be 
excellent  for  this  purpose.  Counterpoise  it  with  weights 
in  the  other  pan.  The  bucket,  of  course,  is  full  of  air. 

Pour  carbon  dioxide  (§  139),  which  is  an  invisible  gas, 
into  the  bucket.  The  carbon  dioxide,  which  is  heavier 
than  air,  pushes  out  the 
air  and  takes  its  place. 
The  side  of  the  balance  to 
which  the  bucket  is  at- 
tached at  once  goes  down 
(Fig.  4).  Tip  the  bucket 
so  that  the  carbon  dioxide 
can  run  out  and  its  place 
is  once  more  taken  by  air. 
The  beam  again  becomes 
horizontal.  Carbon  diox- 
ide evidently  has  a  greater 
weight  than  air. 

Hang  the  bucket,  with 
the  open  end  downward, 
beneath  the  same  pan  of  the  balance.  In  order  that 
an  open  vessel  may  retain  hydrogen  (§  137)  it  must  be 
kept  mouth  downward.  Place  the  mouth  of  the  ves- 
sel containing  the  hydrogen  beneath  one  edge  of  the 
bucket ;  turn  the  vessel  as  if  attempting  to  pour  some- 
thing upward  (Fig.  5).  The  lighter  hydrogen  runs 
up  into  the  bucket  and  pushes  down  the  heavier  air 
until  it  is  entirely  replaced  by  hydrogen.  The  side 
of  the  balance  to  which  the  bucket  is  attached  rises. 


FIG.  4. 


ELEMENTARY   PHYSICS 


Therefore  the  bucket  is  filled  with  a  gas  lighter  than 

air. 

4.  Principles  Involved  in  Pouring  of  Gases — When  water 
is  poured  into  a  tumbler,  it  sinks  to  the  bottom  and 
pushes  out  the  lighter  air.  In  the  same  manner,  when 
carbon  dioxide  is  poured  into  the  paper  bucket,  it  sinks 
to  the  bottom  and  pushes  out  the  air. 

When  a  tumbler  containing  air  is  placed  in  water  open 
end  downward  the  water  is  not  able  to  displace  the  air. 

In  the  same  manner,  when 
a  vessel  containing  hydro- 
gen is  held  with  the  open 
end  downward,  the  heav- 
ier   air    surrounding   the 
vessel  is  not  able  to  push 
out  the  lighter  hydrogen. 
If  the  tumbler  contain- 
ing air  had  been  placed 
in  the  water  with  the  open 
end    upward,    the    water 
would    have    rushed   into 
the   vessel    and    crowded 
out  the  air.     In  the  same 
manner,  as  soon  as  a  ves- 
sel containing  hydrogen  is  turned  until  the  open  end 
faces  upward,  the  heavier  air  runs  into  the  vessel  and 
pushes  out  the  lighter  hydrogen. 

Place  the  tumbler  in  water  so  as  to  permit  the  air  to 
escape.  The  tumbler  is  now  full  of  water.  Invert  the 
tumbler,  and  hold  it  under  the  water  in  this  inverted 
position.  Plunge  a  vessel,  containing  nothing  but  air, 
mouth  downward  into  the  water  and  bring  its  mouth 
beneath  one  edge  of  the  tumbler.  Turn  the  vessel  so 


FIG.  5. 


GASES  AND   LIQUIDS 


that  the  air  can  escape  upward  into  the  tumbler  and 
entirely  fill  it.  In  the  same  manner,  if  the  paper  bucket 
be  suspended  with  the  open  end  downward,  and  a  vessel 
containing  hydrogen,  mouth  end  downward,  be  brought 
near  the  bucket,  so  that  the  mouth  of  the  vessel  be  be- 
neath one  edge  of  the  bucket,  the  vessel  can  be  turned  so 
as  to  permit  the  lighter  hydrogen  to  run  up  into  the 
bucket  and  push  out  the  heavier  air. 

All  gases  are  not  invisible.  Some  of  them  have  very 
characteristic  colors.  Chlorine  has  a  yellowish-green  tint. 
Nitrogen  tetroxide  and  bromine  have  different  shades  of 
reddish  brown.  The  methods  of  obtaining  all  of  these 


FIG. 


FIG.  7. 


gases  can  be  learned  easily  from  any  good  work  on 
chemistry. 

5.  Air  Presses  in  All  Directions. — Hold  a  piece  of  blot- 
ting-paper or  pasteboard  over  the  mouth  of  a  tumbler 
filled  with  water  and  shake  the  tumbler  until  the  pa- 
per is  thoroughly  moistened.  Then  invert  the  tumbler 
(Fig.  6).  What  holds  the  blotter  in  place,  so  that  the  water 
can  not  run  out  ?  In  what  direction  is  the  air  pressing  ? 
Hold  the  tumbler  so  that  the  mouth  will  face  sidewise. 
Does  the  water  run  out  ?  In  what  direction  is  the  air 
pressing  now  ? 

Place  an   argand-lamp   chimney  under  water,  press  a 


6  ELEMENTARY   PHYSICS 

piece  of  blotting-paper  tightly  against  each  end,  and 
remove  the  chimney  from  the  water.  Now  carefully 
remove  the  hands  from  the  blotting-paper  at  both  ends 
of  the  chimney  and  hold  the  chimney  in  a  vertical  posi- 
tion. Why  does  the  water  not  run  out  ?  Hold  the  chim- 
ney in  a  horizontal  position  (Fig.  7).  Hold  it  in  other 
positions.  In  what  direction  is  the  air  pressing  in  these 
different  cases  ?  Notice  that  the  air  is  pressing  on  both 
ends  of  the  chimney.  Indeed,  it  is  pressing  also  on  the 
walls  of  the  chimney,  but  the  walls  are  too  strong  to  give 
evidence  of  this  pressure. 

6.  Effect  of  Inequality  of  Air-Pressure.— The  pressure  of 
the  air  can  be  further  illustrated  by  the  following  experi- 
ment. Tie  a  small  piece  of  sheet 
rubber  over  the  top  of  a  bladder- 
glass  and  place  the  glass  on  the 
plate  of  an  air-pump.  As  soon  as 
part  of  the  air  is  pumped  out  of 
the  glass,  the  pressure  of  the  air 
beneath  the  rubber  is  less  than 
the  pressure  of  the  air  above,  and 
the  rubber  is  forced  down  into  the 
glass  (Fig.  8).  Kemove  the  rub- 
ber and  close  the  opening  at  the 

top  of  the  bladder-glass  firmly  with  the  palm  of  the  hand. 
Pump  out  all  or  nearly  all  of  the  air.  The  hand  now  is 
forced  down  so  strongly  against  the  glass,  that  it  cannot 
be  easily  withdrawn.  What  is  forcing  it  down  ? 

At  the  beginning  of  the  experiment  the  pressure  of  the 
air  within  the  vessel  and  the  pressure  of  the  air  surround- 
ing it  is  the  same.  As  soon  as  a  part  of  the  air  is  pumped 
out  of  the  vessel,  the  pressure  of  the  air  remaining  within 
it  becomes  less  than  the  pressure  of  the  whole  quantity 


GASES   AND   LIQUIDS  7 

of  air  which  was  originally  present.  In  consequence  it 
becomes  less  also  than  the  pressure  of  the  air  surround- 
ing the  vessel.  The  hand  is  forced  down  with  a  force 
equal  to  the  difference  between  the  pressures  exerted  by 
the  air  within  and  without  the  vessel.  When  the  air  is 
once  more  admitted  to  the  bladder-glass  the  pressures 
are  equalized  and  the  hand  may  be  readily  removed. 

Instead  of  a  bladder-glass,  a  small  lantern  globe  may 
be  used.  In  this  case  the  ends  of  the  globe  should  be 
ground  smooth  by  rubbing  them  over  a  piece  of  plate- 
glass  covered  with  a  thin  layer  of  fine  emery  powder 
moistened  with  water. 

Press  most  of  the  air  out  of  the  thin  rubber  bag  at- 
tached to  a  toy  whistle  ;  close  it  tightly,  and  place  it 
beneath  a  bell-jar  (receiver)  on  the  plate  of  an  air-pump. 
The  base  of  the  bell -jar,  where  it  rests  upon  the  pump 
plate,  should  be  rubbed  with  tallow  to  make  it  air-tight. 
Pump  out  of  the  bell-jar  the  air  which  surrounds  the 
rubber  bag.  Why  does  the  rubber  bag  dilate  as  the  air 
around  it  is  gradually  removed  ? 

7.  Vacuum  Fountain.— The  vacuum  fountain  apparatus 
consists  of  a  tall  glass  vessel  open  only  at  the  base. 
From  this  base  a  narrow,  tapering  brass  tube  extends 
several  inches  up  into  the  interior  of  the  vessel,  and  ends 
with  a  small  opening.  The  base  of  the  apparatus  is  pro- 
vided with  a  screw  thread,  so  that  the  vacuum  fountain 
vessel  can  be  fastened,  in  an  upright  position,  on  the 
plate  of  an  air-pump.  The  entrance  to  the  tube  through 
this  base  can  be  opened  or  closed  by  means  of  a  stop- 
cock. 

Fasten  the  vacuum  fountain  vessel  on  the  plate  of  an 
air-pump.  Remove  nearly  all  of  the  air.  Close  the  stop- 
cock. Place  the  lower  end  of  the  vessel  beneath  the  sur- 


8 


ELEMENTAEY   PHYSICS 


face  of  the  water  in  a  glass  jar.     Open  the  stop-cock. 

The  water  is  forced  up  into  the  vessel  in  the  form  of  a 

tiny  spray  like  a  fountain  (Fig.  9). 

Explain.    In  the  figure  the  vessel 

is  represented  as  if  screwed  into  a 

support  which  permits  the  ready 

entrance  of  water  from  beneath. 

This  makes  it  unnecessary  to  hold 

the   apparatus   during  the  latter 

part  of  the  experiment. 

Fill  a  quart  bottle  about  one- 


FlQ.  10. 

third  full  of  water.  Rotate  in  the  flame  of  a  Bunsen 
burner  one  end  of  a  piece  of  glass  tubing,  about  ten 
inches  long,  until  the  opening  at  this  end  becomes  very 
small.  Thrust  the  other  end  through  the  hole  in  a  rubber 
stopper  placed  in  the  mouth  of  a  quart  bottle,  so  that  the 
lower  end  of  the  glass  tube  is  within  a  short  distance  of 


GASES   AND   LIQUIDS  9 

the  bottom  of  the  bottle.  Through  the  glass  tube  blow 
vigorously  into  the  bottle  for  a  short  time,  and  then  re- 
move your  mouth  quickly.  What  causes  the  fountain 
(Fig.  10)  ?  In  the  drawing  the  glass  tube  is  replaced  by 
a  brass  tube  supplied  with  a  stop-cock.  The  stop-cock  is 
closed  before  the  mouth  is  taken  awaj7. 

Compare  in  these  two  experiments  the  relative  press- 
ures of  the  air  within  the  vessel  and  the  air  without  the 
vessel. 

8.  Water  Held  Up  in  a  Tube  by  Air- Pressure.— By  means 
of  the  thumb  close  a  test-tube  filled  with  water,  invert  it, 
and  place  the  mouth  of  the  test-tube  a  short 
distance  beneath  the  surface  of  the  water  in 
a  glass  jar.  Remove  the  thumb.  Why  does 
the  water  not  run  out  of  the  test-tube  (Fig. 
11)  ?  Because  the  pressure  of  the  air  on  the 
surface  of  the  water  in  the  glass  jar  is  suf- 


ficient to  prevent  this. 

Is  there  any  limit  to  the  amount  of  pressure  which  the 
air  exerts?  If  so,  there  must  be  a  limit  to  the  weight 
of  water  which  the  pressure  of  the  air  can  hold  up. 

If,  instead  of  the  short  test-tube,  a  stout  tube  35  feet 
or  more  in  length  and  closed  at  the  top  had  been  used, 
would  the  pressure  of  the  air  on  the  surface  of  the  water 
in  the  vessel  have  been  sufficient  to  hold  up  all  of  the 
water  in  the  tube  ? 

This  has  been  tried,  and  it  has  been  found  that  the 
pressure  of  the  air,  at  sea  level,  is  usually  sufficient  to 
hold  up  about  34  feet  of  water,  but  no  more.  It  can,  of 
course,  hold  up  any  height  less  than  34  feet.  The  water 
in  a  tube  longer  than  34  feet,  therefore,  falls  until  the 
upper  surface  of  the  water  in  the  tube  is  about  34  feet 
above  the  surface  of  the  water  in  the  vessel  (Fig.  12).  If 


10  ELEMENTARY   PHYSICS 

the  pressure  of  the  air  on  the  surface  of  the  water  in  the 
vessel  were  greater,  it  could  hold  up  a  column  of  water 
having  a  greater  vertical  height.  If  the  pressure  of  the 
air  were  less,  the  height  of  the  column  of  water  held  up 
would  be  less  (§  29). 

It  is  evident  that  the  greatest  height  of  water  which 
can  be  held  up  in  a  long  tube  closed  at-  the  top  can  be 
used  to  determine  the  strength  of  pressure  of  the  air  on 
the  water  in  the  vessel.  If  we  know  the  force  with  which 
the  water  in  the  tube  is  pressing  downward,  we  also  know 
the  force  with  which  the  air  must  be  pressing  on  the 
water  in  the  vessel  in  order  to  prevent  the  water  in  the 
tube  from  escaping. 

9.  Amount  of  Pressure  Exerted  by  Air. — If  the  area  of  the 
opening  at  the  base  of  the  tube  is  one  square  inch,  and  if 
the  height  of  the  water  in  the  tube  is  34  feet,  or  408 
inches,  then  the   quantity  of  water  in  the  tube  is  408 
cubic  inches.     It  has  been  determined  by  actual  weighing 
that  the  weight   of  one  cubic  inch  of  water  is   .036  + 
pounds.     Then  the  weight  of  the  total  quantity  of  water 
present  in  the  tube  is  .036  x  408  =  14.688  (14.7)  pounds. 
Therefore  the  water  in  the  tube  presses  downward  with 
the  force  of  14.7  pounds  on  the  square  inch  of  water  at 
the  base  of  the  tube,  and  the  air  evidently  presses  down- 
ward on  the  surface  of  the  water  in  the  vessel  with  the 
same  force  (§  12). 

10.  Height  of  Column  Dependent  upon  Density  of  Liquid. 
—In  the  case  of  liquids  heavier  than  water,  the  greatest 
height  of  the  liquid  which  can  be  held  up  by  this  air 
pressure  of  14.7  pounds  on  each  square  inch  of  surface 
must  necessarily  be  less  than  34  feet.     Since  mercury 
weighs  13.6  times  as  much  as  an  equal  volume  of  water, 
the  highest  column  of  m,ercury  which  can  be  held  up  by 


GASES   AND   LIQUIDS 


11 


the  air-pressure  of  147  pounds,  is  only  TJ:^  or  T1^°F  as  high 
as  the  highest  column  of  water  held  up  by  the  same  air 
pressure.  This  height  is  TW  of  408  =  30  inches  (Fig.  12). 

In  order  to  show  the  height  of  mer- 
cury which  can  be  held  up  by  air-press- 
ure, take  a  stout  glass  tube  more  than 
30  inches  long  and  closed  at  one  end ; 
fill  it  with  mercury,  close  the  open  end 
with  the  finger,  and  dip  this  end  under 
the  surface  of  some  mercury  in  a  small 
vessel.     A  broad  salt-cellar  is  very  con- 
venient.    Eemove  the  fin- 
ger.    If  the  tube  be  held 
vertically,     mercury    will 
run  out  until  the  upper 
surface  of  the  mercury  in 
the  tube  does  not  rise  over 
30  inches  above  the  level 


n 


the 


Uft. 


Mercury 


Water 


FIG.  12. 


of    the    mercury 
vessel. 

11.    Height    of    Column 

Measured  Vertically.  —  If  the  tube  be  inclined,  mercury 
runs  toward  the  closed  end  of  the  tube,  so  that  a  greater 
quantity  of  mercury  is  held  up  within  the  tube.  If,  how- 
ever, the  vertical  distance  from  the  surface  of  the  mer- 
cury within  the  tube  to  the  surface  of  the  mercury  in  the 
vessel  be  measured,  it  will  be  found  to  be  still  30  inches 
(Fig.  13).  In  other  words,  an  air-pressure  of  14.7  pounds 
per  square  inch  can  hold  up  mercury  to  the  vertical  height 
of  30  inches,  irrespective  of  the  variation  in  the  quantity 
of  mercury  held  up.  If  tubes  of  very  different  diameters 
be  taken,  it  will  be  found  that  mercury  can  be  held  up  to 
the  same  vertical  height  of  30  inches  in  all  of  these  tubes; 


ELEMENTARY   PHYSICS 


although,  of  course,  the  quantity  of  mercury  in  the  wider 
tubes  greatly  exceeds  that  in  the  narrow  ones. 

If  now  tubes,  variously  bent  and  varying  in  diameter, 
be  dipped  in  vessels  exposing  different  amounts  of  surface 
of  mercury,  it  will  be  found  that  the  same  air-pressure 
can  always  hold  up  the  same  vertical  height  of  mercury, 
irrespective  not  only  of  the  form  of  the  tube  and  its  di- 
ameter, but  also  of  the  area  of  the  mercury  exposed  in  the 
vessel  into  which  the  base  of  the  tube  is  dipped.  There- 
fore, in  determining  the  pressure  of  the  air  by  means  of 

the  column  of  mercury  held  up 
in  a  barometer,  it  is  customary 
to  state  that  the  pressure  of  the 
air  is  equivalent  to  the  pressure 
of  a  column  of  mercury  a  cer- 
tain number  of  inches  or  cen- 
timeters in  height,  and  no  men- 
tion is  made  of  the  diameter  of 
the  tube,  the  weight  of  the  mer- 
cury which  it  contains,  or  of  the 
area  of  the  mercury  exposed  in 
the  vessel.  In  other  words,  in- 
stead of  stating  that  the  pressure  of  the  air  is  14.7  pounds 
per  square  inch  it  is  often  stated  that  the  pressure  of  the 
air  is  equal  to  30  inches. 

12.  Amount  of  Air-Pressure  on  One  Square  Inch  at  Sea 
Level, — In  the  experiments  just  described,  the  pressure 
of  the  air  is  not  14.7  pounds  on  the  entire  surface  of  the 
mercury  in  the  vessel,  but  14.7  pounds  on  each  square 
inch  of  that  surface.  An  explanation  of  the  reasons  for 
this  fact  involves  a  knowledge  of  Pascal's  Law  (§53),  and 
had  best  be  deferred  until  that  law  has  been  studied. 
The  fact  that  the  pressure  of  air  is  14.7  pounds  on  each 


FIG.  13. 


GASES   AND   LIQUIDS 


13 


FIG.  14  A. 


square  inch  can  be  demonstrated  without  a  knowledge  of 
Pascal's  Law  by  the  following  experiment. 

Suspend  from  a  strong  support,  seven  or  more  feet  above 
the  floor  of  the  room,  a  short  but  broad  brass  cylinder, 
closed  at  the  top,  with  the  open  end 
facing  downward  (Fig.  14  A).  A  mov- 
able brass  plate,  having  an  area  of 
26  square  inches,  fits  air-tight  at  all 
levels  within  this  cylinder.  A  small 
brass  tube  at  the  top  of  the  cylinder 
permits  the  escape  of  most  of  the  air 
when  the  plate  is 
thrust  to  its  highest 
position  within  the  cylinder.  A  brass 
handle  is  attached  to  the  lower  surface 
of  the  movable  plate.  From  the  handle 
may  be  suspended,  by  means  of  a  rope, 
a  board  such  as  is  used  for  swings. 
Thrust  the  brass  plate  as  far  up  in  the 
brass  cylinder  as  possible  and  permit 
the  air  to  escape.  With  a  pinch-cock 
close  the  rubber  tube  slipped  over  the 
top  of  the  brass  exit  tube  by  means  of 
which  the  air  escapes,  and,  using  boys, 
place  as  great  a  weight  upon  the  swing- 
board  as  the  apparatus  will  safely  hold 
(Fig.  14  B).  This  total  weight  will  be 
found  to  equal  or  exceed  260  pounds. 
The  boys  are  evidently  held  up  by  the 
excess  of  pressure  of  the  air  against  the  lower  surface  of 
the  plate,  as  compared  with  the  pressure  of  the  small 
quantity  of  air  remaining  in  the  cylinder  above  this 
plate.  This  demonstrates  that  the  pressure  of  the  air  on 


FIG.  14  B. 


14  ELEMENTAKY   PHYSICS 

26  square  inches  either  equals  or  exceeds  260  pounds. 
Therefore  the  pressure  of  air  on  each  square  inch  equals 
or  exceeds  10  pounds. 

If  a  cylinder  of  larger  diameter  and  a  plate  of  larger 
area  had  been  used,  it  would  have  been  found  that  a  cor- 
respondingly greater  weight  could  have  been  held  up 
by  the  air.  By  this  means  it  can  be  demonstrated  that 
there  is  a  definite  pressure  of  the  air  on  every  square  inch, 
and  that  the  total  pressure  of  the  air  depends  upon  the 
total  area  of  the  surface  exposed.  If  all  the  air  above 
the  plate  in  the  cylinder  had  been  removed  by  an  air- 
pump,  the  total  pressure  of  the  air  on  the  lower  surface 
of  the  plate  would,  at  sea  level,  have  been  found  to  be 
more  nearly  14.7  pounds  on  each  square  inch. 

13.  Variation  in  Air-Pressure  Due  to  Elevation — A  small 
quantity  of  air  has  only  a  slight  weight,  but,  since  the 
atmosphere  extends  for  a  distance  of  more  than  100  miles 
above  the  earth,  the  total  quantity  of  air  overlying  any 
square  inch  of  the  earth's  surface  is  very  great,  and  the 
pressure  produced  by  that  quantity  of  air  on  one  square 
inch  at  the  surface  of  the  earth  must  be  considerable. 
The  pressure  of  the  atmosphere  upon  any  square  inch 
of  surface  depends  upon  the  total  quantity  of  air  directly 
overlying  that  surface.  This  quantity  will  be  greater 
at  the  level  of  the  sea  than  on  the  summit  of  a  high 
mountain,  in  the  bottom  of  a  valley  than  at  the  top  of 
a  hill,  at  the  base  of  a  building  than  in  one  of  its  upper 
stories. 

If  the  amount  of  pressure  of  the  atmosphere  on  any 
surface  be  proportional  to  its  height  above  sea  level,  it  is 
evident  that  the  pressure  of  the  atmosphere  upon  that 
surface  can  be  used  to  determine  its  elevation  above  the 
sea.  This  is  so  nearly  true,  when  corrections  are  made 


GASES   AND   LIQUIDS  15 

for  temperature,  especially  when  within  moderate  dis- 
tances from  the  surface  of  the  earth,  that  the  degree  of 
air-pressure  on  a  square  inch  of  surface  at  any  locality 
has  often  been  used  as  a  means  of  determining 
its  distance  above  sea  level.  The  height  of  most 
mountains  has  been  determined  in  this  manner 
(§  100).  This  method  is  especially  useful  where  it 
is  either  too  difficult  or  too  expensive  to  deter- 
mine the  elevation  by  means  of  the  more  accu- 
rate methods  used  by  the  surveyor. 

14.  The  Barometer. — The  barometer  is  the  in- 
strument used  in  measuring  atmospheric  press- 
ure. There  are  two  kinds  of  barometers  in  ordi- 
nary use,  the  mercurial  and  the  aneroid. 

A  glass  tube  usually  34  inches  long,  with  a 
small  bore  (internal  diameter)  and  very  thick 
walls,  and  closed  at  one  end,  is  filled  with  mer- 
cury. The  open  end  is  held  shut  by  means  of 
the  thumb,  and  in  this  condition  is  inserted  be- 
neath the  surface  of  some  mercury  placed  in  a 
small  sized  cup.  The  air-pressure  on  the  mer- 
cury in  the  cup  is  not  sufficient  to  hold  up  or 
balance  the  entire  vertical  height  of  mercury  in 
the  tube.  It  will  be  remembered  (§  10)  that  the 
height  of  mercury  held  up  by  the  average  air- 
pressure  at  sea-level,  is  only  30  inches  (Fig.  15). 
Therefore  the  mercury  in  a  barometer-tube  34 
inches  long  will  fall  at  least  4  inches  at  sea-level. 

If  the  barometer  be  carried  to  elevations  above    M|I< 

sea-level,  the  column  of  mercury  in  the  tube 
gradually  falls  below  30  inches  at  the  rate  of  FlG-  n- 
1  inch  for  a  rise  in  elevation  of  about  910  feet.  The 
height  of  the  column  of  mercury  in  a  barometer-tube 


16 


ELEMENTARY   PHYSICS 


at  any  locality  may  therefore  be  utilized  to  determine  its 
elevation  above  sea-level. 

In  order  to  determine  approximately  the  height  of  the 
top  of  a  mountain  above  its  base  (not  above  sea-level), 
ascertain  the  height  of  the  column  of  mercury  in  a 
barometer-tube  both  at  the  top  of  the  mountain  and  at  its 
base  and  multiply  by  910  the  difference  between  the  two 
heights  expressed  in  inches.  The  product  will  be  the 
height  of  the  mountain  expressed  in  feet.  The  heights 
of  mountains  are,  however,  given  in  geographies  as  so 
many  feet  above  sea-level. 

15.  The  Barometer  as  an  Indicator  of  Changes  in  Weather. 
—Variations  in  the  height  of  the  barometer  are  also  due 

to  atmospheric   conditions  which  cause  changes  in  the 
weather.     Such  variations  are   especially  noticeable   at 

times  preceding  violent 
storms.  In  general  it  may 
be  said  that  a  rapid  rise  of 
the  mercury  in  the  barom- 
eter indicates  fair  weather 
and  a  rapid  fall  indicates 
stormy  weather,  including 
probably  rain,  sleet,  or  snow. 
A  steady  barometer  indi- 
cates a  continuance  of  exist- 
ing conditions.  The  use  of 
the  barometer  in  determin- 
ing variations  in  the  weather 
can  be  studied  with  profit  in 
some  book  on  meteorology.* 

16.  Aneroid     Barometer.  —  The      aneroid     barometer 
(Fig.  16)  consists  of  a  round  flat  box  from  which  most  of  the 

*  Davis,  Elementary  Meteorology.     Ginn  &  Co. 


FIG.  16. 


GASES   AND   LIQUIDS  17 

3 

air  has  been  pumped.  It  is  made  of  metal  so  thin  that 
any  increase  in  the  pressure  of  the  air  on  the  exterior  can 
slightly  press  down  its  upper  surface.  When  the  press- 
ure of  the  air  is  decreased,  the  top  of  the  box  rises  again. 
The  amount  of  motion  of  the  top  of  the  box  is  so  slight 
that  it  cannot  be  readily  noticed.  It  is,  however,  made 
evident  by  a  little  mechanism  attached  to  the  top  of  the 
box,  which  causes  the  end  of  a  small  pointer  to  move  a 
considerable  distance  around  the  face  of  a  dial  covering 
the  box,  even  when  the  motion  of  the  thin  top  of  the  box 
is  but  slight.  The  dial  resembles  very  much  the  face  of 
a  watch,  but  is  supplied  with  two  sets  of  figures.  The 
inner  one  of  these  indicates  the  height  in  inches  at  which 
a  mercurial  barometer  would  stand  under  similar  con- 
ditions of  air-pressure.  The  outer  set,  often  placed  on  a 
movable  ring,  is  used  to  indicate  approximately  in  feet 
the  level  of  the  observer  above  the  sea  (Fig.  16). 

If  there  has  been  a  change  of  weather  between  the 
observations  at  two  localities,  the  barometer  cannot  be 
used  to  determine  with  any  reasonable  degree  of  accu- 
racy their  difference  in  elevation. 

17.  Variation  of  Density  of  Air  with  Altitude.— If  the 
opening  of  a  pocket  bicycle-pump  is  closed  by  means  of 
a  finger  and  the  plunger  is  then  pushed  inward,  the  air 
within  the  pump  is  forced  by  the  plunger  into  a  smaller 
space,  or,  is  compressed-  The  greater  the  pressure  ex- 
erted by  the  plunger  upon  the  air,  the  smaller  is  the 
volume,  or  the  amount  of  space  occupied.  The  air  which 
has  been  crowded  together  into  smaller  space  is  said  to 
be  more  dense.  The  density  of  air  within  any  space  varies 
with  the  amount  of  air  crowded  into  that  space. 

The  upper  part  of  the  atmosphere  rests  upon  that 
which  is  below.  It  possesses  weight,  and,  because  of 


18 


ELEMENTARY   PHYSICS 


this  weight,  it  pushes  down  upon  and  compresses  the 
air  beneath  it.  On  this  account  the  lower  parts  of  the 
atmosphere  are  more  compressed  than  the  upper  parts. 


FIG.  17. 


Consequently,  the  greater  quantity  of  the  air  is  found 
near  the  surface  of  the  earth.  At  an  elevation  of  1J 
miles  above  the  earth,  the  density  of  the  air  is  less  than 
f  of  the  density  at  the  level  of  the  sea,  at  3J  miles  it  is 


GASES   AND   LIQUIDS  19 

about  §  of  the  density  &t  sea  level,  and  at  6  miles  it 
is  less  than  J  as  great  (Fig.  17).  Half  of  the  entire  atmos- 
phere is  found  crowded  together  within  3J  miles  of  the 
sea  level,  and  about  f  of  the  atmosphere  is  found  within 
6  miles  of  sea  level.  It  is  evident  that  100  miles  above 
the  earth's  surface  the  air  must  be  exceedingly  thin  or 
rare. 

18.  Total  Pressure  Exerted  by  Atmosphere  on  Human 
Body. — The  pressure  of  the  air  at  sea  level  is  about  14.7 
pounds  per  square  inch  (§  12).  Every  square  inch  of  the 
surface  of  the  human  body  is  subjected  to  a  pressure  of 
14.7  pounds.  Since  the  total  surface  of  the  body  is  equal 
to  hundreds  of  square  inches,  the  total  atmospheric  press- 
ure upon  the  body  amounts  to  a  number  of  tons.  It  has 
been  calculated  to  exceed  15  tons,  or  30,000  pounds,  for 
an  average-sized  person.  If  the  body  were  not  saturated 
with  blood,  lymph,  and  other  liquids,  it  could  not  with- 
stand this  enormous  pressure.  But  our  bodies  are  con- 
structed so  as  to  be  most  healthy  when  under  this  press- 
ure. Without  it  we  feel  uncomfortable.  On  the  tops 
of  high  mountains,  where  the  pressure  is  much  reduced, 
breathing  becomes  more  rapid  and  more  laborious,  the 
pulse  beats  more  quickly,,  painful  headaches  and  vomiting 
often  result,  and  any  physical  exercise  causes  great  fatigue. 

Fish  living  at  the  bottom  of  the  sea  are  subjected  to 
enormous  pressure.  Nevertheless,  fish  adapted  to  those 
great  depths  live  there  comfortably.  In  the  fish  here 
figured  (Fig.  18)  the  eyes  are  very  large  in  proportion  to 
the  size  of  the  fish,  and  the  lower  half  of  the  body  is  cov- 
ered with  phosphorescent  spots.  This  enables  the  fish 
to  see  at  great  depths.  "Were  the  great  pressure  to  which 
they  are  accustomed  diminished  to  any  very  consider- 
able extent,  they  would  suffer  great  pain,  possibly  death. 


20 


ELEMENTARY   PHYSICS 


Alexander  Agassiz  says,  "  In  fish  brought  up  from  deep 
water,  the  swimming-bladder  often  protrudes  from  the 
mouth,  the  eyes  are  forced  out  of  their  sockets,  the  scales 


FIG.  is. 

have  fallen  off,  and  they  present  a  most  disreputable  ap- 
pearance." 

Just  as  deep-sea  fish  need  the  greater  pressures  of  the 
deeper  water,  so  we  need  the  greater  pressures  of  the  air 
found  at  lower  altitudes  on  the  earth's  surface. 

19.  Balancing  of  Liquids  in  U-tubes,  —  Pour  mercury 
into  a  U-tube  of  uniform  bore  with  arms  at  least  20 
inches  long,  until  the  mercury  rises  to  a 
height  of  about  4  inches  in  one  arm.  What 
is  the  height  of  the  mercury  in  the  other 
arm  ?  Pour  additional  mercury  into  the 
same  arm  of  the  tube  until  the  mercury  on 
this  side  rises  to  a  height  of  about  6  inches. 
The  mercury  rises  to  the  same  height  in  the 
other  arm,  and  the  two  columns  of  mercury 
exactly  balance  each  other. 

Now  pour  3  ounces  of  water  on  top  of  the 
mercury  in  the  right  arm  (Fig.  19).  What  effect  is  pro- 
duced upon  the  level  of  the  mercury  in  the  other  arm  ? 


FIG.  19. 


GASES   AND   LIQUIDS 


21 


Although  the  liquids  in  the  two  arms  do  not  rise  to  the 
same  level,  the  wrater  and  the  mercury  remaining  in  the 
right  arm  of  the  tube  together  bal- 
ance the  total  quantity  of  mercury 
which   is   now   present  in  the  left 
arm.     The   mercury   remaining   in 
the  right  arm  balances  only  that 
part  of  the  mercury  in  the  other 
arm  which  does  not  rise  above  the 
level  of  the  mercury  in  the  right 
arm.     That  part  of  the  mercury  in 
the  left  arm  which  rises  above  this 
level,    is    balanced    by    the   water 
which  has    been  poured  into  the 
right  arm.     If  the  pressure  exerted 
by  the  water  is  3  ounces,  then  the    |f 
pressure  exerted   by  that   part   of    || 
the  mercury  in  the  left  arm  which    s| 
is  balanced  by  the  water  is  also  3    f  5 
ounces.  ** 


20.  Boyle's  Law :  Apparatus,  — 
Pressure  affects  the  volume  of 
gases.  To  illustrate  the  relation 
between  pressure  and  volume,  the 
following  apparatus  has  been  de- 
vised (Fig.  20).  A  long  glass  tube 
of  uniform  bore  is  bent  at  the  mid- 
dle, so  as  to  resemble  a  very  much 
elongated  letter  U.  The  Jube  is 
mounted  vertically,  with  the  open 
ends  upward.  The  parallel  arms 

should  have  a  length  of  180  centimeters,  or  about  71 
inches.     A  glass  stop-cock  is  inserted  100  centimeters,  or 


FIG.  20. 


22  ELEMENTARY   PHYSICS 

about  40  inches,  above  the  base  of  the  left  arm,  in  order 
to  close  this  arm  whenever  necessary.  A  short  tube  with 
a  second  stop-cock  is  attached  to  the  base  of  the  U-tube 
for  convenience  in  emptying  the  apparatus  at  the  close 
of  the  experiment. 

Close  the  stop-cock  at  the  base  of  the  tube  and  open  the 
stop-cock  in  the  left  arm.  Pour  in  mercury  until  it  rises 
slightly  above  the  curved  part  at  the  base  of  the  two  arms 
of  the  tube.  The  mercury  rises  to  the  same  level  on  both 
sides.  Since  the  air  in  the  two  arms  is  subjected  to  the 
same  atmospheric  pressure,  the  density  of  the  air  in  both 
is  the  same.  Close  the  stop-cock  in  the  left  arm.  This 
neither  increases  nor  diminishes  either  the  density  or  the 
pressure  of  the  air  in  that  arm.  It  merely  confines  the 
air  already  present.  Since  the  pressure  of  the  confined 
air  remains  the  same,  the  surface  of  the  mercury  remains 
at  the  same  level  in  the  two  arms.  In  other  words,  while 
the  total  atmosphere  is  pressing  with  a  certain  force 
upon  the  mercury  at  the  base  of  the  right  arm,  the  small 
quantity  of  air  confined  in  the  left  arm  is  pressing  with 
equal  force  on  the  mercury  there. 

21.  Boyle's  Law:  Experiment— Determine  the  height 
of  the  mercury  in^the  barometer  at  the  time  of  the  experi- 
ment. 

Pour  mercury  into  the  right  arm  of  the  apparatus 
used  in  the  preceding  experiment  until  the  difference  be- 
tween the  levels  of  the  mercury  in  the  two  arms  is  equal 
to  the  height  of  mercury  in  the  barometer  (Fig.  20).  The 
mercury  now  present  in  the  left  arm  balances  only  an 
equal  height  of  the  mercury  in  the  right  arm.  But  the 
mercury  in  the  right  arm  above  this  level  is  balanced  by 
the  increased  pressure  now  exerted  by  the  air  confined 
in  the  left  arm.  This  increased  pressure  is  caused  by 


GASES  AND   LIQUIDS  23 

the  fact  that  the  air  confined  in  the  left  arm  has  been  con- 
densed into  a  smaller  space  and  therefore  presses  back 
with  greater  force.  Since  that  part  of  the  mercury  in  the 
right  arm  which  rises  above  the  level  of  the  mercury  in 
the  left  has  the  same  height  as  the  mercury  held  up  in 
the  barometer,  it  exerts  the  same  relative  pressure  as  the 
atmosphere  at  the  time  of  the  experiment. 

At  the  beginning  of  the  experiment  the  pressure  of 
the  air  confined  in  the  left  arm  balanced  the  pressure  of 
the  total  atmosphere  pressing  down  the  open  arm.  The 
air  confined  in  the  left  arm  therefore  also  exerted  a 
pressure  of  one  atmosphere.  Now  the  pressure  of  the 
air  in  the  closed  arm  balances  both  the  pressure  of  the 
air  in  the  open  arm  and  the  excess  of  mercury  in  that 
arm,  a  total  pressure  of  two  atmospheres.  Therefore, 
when  the  difference  between  the  levels  of  the  mercury  in 
the  two  arms  is  equal  to  the  height  of  the  mercury  in  the 
barometer,  the  pressure  of  the  air  confined  in  the  left  arm 
is  equal  to  two  atmospheres. 

Observe  that  the  air  in  the  left  arm  occupies  now  only 
one-half  of  its  original  volume.  In  other  words,  its  pres- 
ent density  is  double  its  original  density.  This  demon- 
strates that  : 

When  air  is  reduced  to  one-half  of  its  original  volume, 
or,  when  its  density  is  doubled,  it  exerts  double  its  original 
pressure. 

If  mercury  should  be  added  to  the  right  arm  until  the 
excess  of  mercury  in  that  arm  is  equal  to  twice  the 
height  of  the  mercury  in  the  barometer,  then  a  total  of 
three  atmospheres  of  pressure  would  be  exerted  upon  the 
balanced  part  of  mercury  in  the  lower  part  of  the  U- 
tube.  The  air  confined  in  the  left  arm  in  that  case,  how- 
ever, would  occupy  only  one-third  of  its  original  volume, 


THE 


24  ELEMENTARY   PHYSIOS 

in  other  words  would  possess  three  times  its   original 
density  and  exert  three  times  its  original  pressure. 

From  a  series  of  such  experiments  the  following  rules 
can  be  derived : 

The  pressure  of  a  gas  varies  inversely  as  its  volume  and 
directly  as  its  density. 

The  volume  of  a  gas  varies  inversely  as  the  pressure  which 
it  exerts,  or  which  is  exerted  upon  it. 

The  density  of  a  gas  varies  directly  as  the  pressure  it  exerts, 
or  the  pressure  (^exerted  upon  it. 

These  rules  are  often  expressed  in  a  condensed  form 
known  as  Boyle's  Law. 

What  is  the  meaning  of  the  mathematical  expressions 
"  varies  inversely  "  and  "  varies  directly  "  ? 

22.  Valves. — One  of  the  common  forms  of  valve  con- 
sists of  a  piece  of  leather  which  is  caused,  by  its  weight, 
to  rest  on  or  against  an  opening.  One  edge  of  this  piece 
of  leather  is  attached,  to  prevent  it  from  shifting  its  posi- 
tion. When  the  pressures  on  both  face  and  back  of  the 
valve  are  equal,  the  weight  of  the  valve  keeps  it  closed. 
If  the  pressure  against  the  face  of  the  valve  (the  side 
in  contact  with  the  opening)  is,  in  any  manner,  made 

greater  than  the  weight 
of  the  valve  plus  what- 
ever pressure  may  be 
exerted  upon  its  back, 
the  valve  is  pushed 
open  (Fig.  21,  A).  When 
the  weight  of  the  valve 

plus  the  pressure  on  its 
FIG.  21. 

back  is  greater  than  the 

pressure  on  the  face,  the  valve  is  pushed  against  the 
opening  (Fig.  21,  B).     In  the  latter  case,  when  the  differ- 


GASES   AND   LIQUIDS  25 

ence  in  pressures  is  considerable,  the  valve  is  closed  with 
sufficient  force  to  prevent  any  gas  or  liquid  from  passing 
through  the  opening. 

The  pressures  acting  upon  valves  are  usually  caused  by 
gases  or  liquids,  usually  air  or  water.  In  the  case  of  a 
large  valve,  such  as  is  used  in  the  various  kinds  of  pumps, 
the  valve  is  often  stiffened  by  a  piece  of  wood  or  iron 
fastened  to  its  back. 

23.  Piston. — A  piston  is  a  round  disk  or  plate  which, 
although  it  can  be  moved  back  and  forth  in  a  cylinder, 
fits  so  tightly  against  the  inner  wall  of  the  cylinder  that 
no  air  can  pass  between  the  piston  and  the  wall.     The 
rod  by  means  of  which  the  piston  is  moved  is  called  the 
piston-rod.     In  some  pumps  an  opening  passes  vertically 
through  the  piston,  at  or  near  its  centre,  and  this  is  cov- 
ered by  an  outward-opening  valve  in  order  to  permit  the 
passage  of  air  in  an  outward  direction  only  (Fig.  22,  A,  B). 
When  the  piston  is  not  provided  with  such  an  opening, 
all  passage  of  air  from   one  side  of   the  piston  to  the 
other  is  prevented  (Fig.  22,  C,  D).     In  this  case  the  out- 
ward-opening valve  is  placed  within  a  tube  leading  out 
laterally  from  the  lower  part  of  the  cylinder. 

In  both  kinds  of  pump  there  is  an  inward-opening 
valve  which  closes  the  opening  at  the  base  of  the  cylin- 
der. Where  it  is  desired  to  force  the  piston  down  so  far 
that  it  will  come  in  direct  contact  with  the  base  as  in 
the  air-pump,  this  inward-opening  valve  is  placed  in  a 
little  depression  formed  at  the  upper  end  of  the  opening 
passing  through  the  base. 

24.  Pumps. — Before  either  form  of  pump  is  set  in  opera- 
tion, the  same  atmospheric  pressure  is  found  at  all  points 
within  and  without  the  pump.     When  the  piston  is  forced 
downward,  the  air  enclosed  within  the  lower  part  of  the 


ELEMENTARY   PHYSICS 


cylinder  below  the  piston,  is  compressed.  The  air  which 
has  been  compressed  exerts  a  greater  pressure  than  the 
atmosphere  without.  In  consequence,  the  inward-open- 
ing- valve,  wherever  placed,  is  forced  shut,  and  the  out- 
ward-opening- valve  is  forced  open  (Fig.  22,  A,  C).  Air 
continues  to  escape  through  the  opened  valve,  until  the 
pressure  of  the  air  remaining  in  the  cylinder  below  the 
piston  is  reduced  so  nearly  to  the  pressure  of  the  atmos- 
phere without  that  it  no  longer  holds  up  the  valve.  Then 
the  valve  drops  on  account  of  its  own  weight. 


-Ordinary  afr 


Fia.  22  A. 


FIG.  22  B. 


When  the  piston  is  raised,  the  air  remaining  in  the 
lower  part  of  the  cylinder  is  given  room  for  expansion. 
As  it  expands  its  density  becomes  less,  and  it  then  exerts 
less  pressure  than  the  atmosphere  without.  In  conse- 
quence, the  outward-opening  valve  is  held  shut  and  the 
inward-opening  valve  is  forced  open  (Fig.  22,  B,  D).  The 
air  from  the  outside  then  enters  the  cylinder.  When  the 
pressure  of  the  air  within  the  cylinder  has  increased  to 


GASES   AND   LIQUIDS 


such  an  extent  that  the  pressure  of  the  outside  atmos- 
phere is  no  longer  sufficient  to  hold  open  the  inward- 
opening  valve,  the  valve  drops. 

When  the  piston  is  lowered  the  second  time,  the  air 
just  admitted  to  the  lower  part  of  the  cylinder  is  com- 
pressed in  its  turn,  and  is  forced,  in  the  manner  already 
described,  through  the  outward-opening  valve.  The  oper- 
ation of  the  piston,  therefore,  causes  an  inflow  of  air 
through  the  inward-opening  valve  at  each  rise  of  the 


FIG.  22  C. 


FIG.  22  D. 


piston,  followed  by  an  outflow  through  the  outward- 
opening  valve  at  each  descent.  In  consequence,  air 
passes  in  an  intermittent  stream  through  the  inward- 
opening  valve  into  the  lower  part  of  the  cylinder,  and 
then  out  through  the  outward-opening  valve.  In  order 
to  pump  air  out  of  any  vessel  it  is  necessary  to  connect 
the  vessel  with  the  inward-opening  valve.  In  order  to 
pump  air  into  any  vessel  the  vessel  must  be  connected 
with  the  outward-opening  valve. 


28 


ELEMENTARY   PHYSICS 


25.  Air-Pump:  Use  of  Pump-Plate  and  Bell-jar.— In  a 
pump  used  for  pumping-  out  air  (Fig.  23)  the  opening 
beneath  the  valve  at  the  base  of  the  cylinder  is  connected 
by  means  of  a  tube  with  a  second  opening  which  passes 
up  through  the  centre  of  a  flat  circular  plate  of  iron  or 
brass.  This  plate  is  called  the  pump-plate.  The  inner 
wall  of  the  opening  through  the  plate  is  supplied  with 


FIG.  23. 

a  spiral  screw  thread,  so  that  it  is  possible  to  screw  ves- 
sels, air-tight,  into  the  top  of  this  plate.  Any  hollow 
vessel  screwed  into  the  pump-plate  is  then  connected  to 
the  pump  by  means  of  the  tube  already  mentioned.  For 
this  reason  the  base  of  the  hollow  sphere  used  to  deter- 
mine the  weight  of  air  (§  2),  and  the  lower  end  of  the  brass 
tube  of  the  vacuum  fountain  vessel  (§  7)  are  screwed  into 


GASES   AND   LIQUIDS  29 

the  opening  in  the  pump-plate,  when  it  is  desired  to  re- 
move the  air. 

For  the  purpose  of  exhibiting  experiments  demanding 
the  removal  of  part  or  all  of  the  air  surrounding  the  ap- 
paratus, tall,  cylindrical  glass  vessels,  called  bell-jars  or 
receivers,  are  placed  on  the  pump-plate.  In  order  to  pre- 
vent air  from  entering  between  the  pump-plate  and  the 
base  of  the  bell-jar,  the  lower  edges  of  the  jar  are  ground 
perfectly  flat,  so  that  they  may  rest  evenly  upon  the  plate. 
The  entrance  of  the  air  may  be  prevented  still  more  ef- 
fectually by  coating  the  base  of  the  bell-jar  with  a  thin 
layer  of  tallow,  and  then  forcing  the  jar  against  the 
pump-plate  with  a  slightly  twisting  motion.  Softer  fats 
and  oils  are  serviceable  but  are  not  as  effective. 

As  soon  as  any  considerable  portion  of  the  air  within 
the  bell- jar  has  been  removed,  the  pressure  of  the  air 
within  is  diminished  so  much  that  the  exterior  atmos- 
phere presses  the  jar  against  the  plate  with  sufficient 
force  to  make  it  difficult,  or,  in  the  case  of  a  large  bell- jar, 
practically  impossible  to  remove  the  jar  from  the  plate. 
Under  these  conditions  very  little  air  can  enter  between 
the  base  of  the  bell-jar  and  the  pump-plate. 

When  it  is  desired  to  remove  the  jar,  air  must  be  ad- 
mitted in  some  manner  to  the  interior  of  the  bell- jar. 
This  is  provided  for  in  some  air-pumps  by  placing,  on 
one  side  of  the  tube  connected  with  the  base  of  the  pump- 
plate,  a  valve  which  can  be  opened  whenever  necessary. 
In  the  case  of  the  air-pump  illustrated  by  the  figure  air 
may  be  admitted  between  the  pump  and  the  manometer 
at  an  opening  usually  closed  by  means  of  a  screw  plug. 

26.  Action  of  Air-Pump.— At  the  beginning,  the  press- 
ures of  the  air  in  the  receiver,  and  of  the  air  in  the 
cylinder  of  the  pump,  both  below  and  above  the  piston, 


30  ELEMENTARY  PHYSICS 

are  the  same.  In  a  good  air-pump,  the  piston,  when 
forced  to  its  lowest  position,  fits  so  snugly  against  the 
base,  that  but  a  very  slight  quantity  of  air  can  remain  in 
the  lower  part  of  the  cylinder.  When  the  piston  is  raised, 
this  slight  quantity  of  air  expands  so  many  hundred  times, 
in  occupying  the  enormously  increased  space  now  pres- 
ent below  the  piston,  that  its  pressure  becomes  almost 
nothing.  At  the  moment  the  piston  begins  to  ascend, 
the  pressure  of  the  exterior  atmosphere  closes  more 
tightly  the  outward- opening  valve  in  the  piston,  while 
some  of  the  air  within  the  bell-jar  and  within  the  con- 
necting tube  forces  its  way  through  the  inward-opening 
valve  into  the  lower  part  of  the  cylinder  (Fig.  23).  When 
the  piston  is  forced  downward,  the  air  which  has  entered 
the  lower  part  of  the  cylinder  beneath  the  piston  is  com- 
pressed ;  this  enables  it  to  close  the  inward-opening  valve 
connecting  with  the  passage  to  the  bell-jar  and  to  force  its 
way  out  through  the  outward-opening  valve  in  the  piston. 
Each  upward  and  downward  movement  of  the  piston  is 
a  repetition  of  these  processes.  Each  upward  stroke  per- 
mits more  air  to  flow  from  the  bell-jar  into  the  lower  part 
of  the  pump-cylinder,  and  each  downward  stroke  forces 
most  of  this  air  to  pass  to  the  space  above  the  piston. 
In  consequence,  the  quantity  of  air  remaining  in  the  bell- 
jar  steadily  diminishes.  Since,  in  a  good  air-pump,  the 
slight  quantity  of  air  remaining  in  the  lower  part  of  the 
cylinder  exerts  almost  no  pressure  when  the  piston  is 
raised,  the  air  in  the  bell-jar  will  continue  to  force  its 
way  into  the  pump  as  long  as  it  is  able  to  overcome  the 
weight  of  the  lower  valve  at  the  base  of  the  cylinder. 
The  degree  of  exhaustion  which  can  be  secured  by  a 
good  air-pump  is,  therefore,  determined  to  a  large  ex- 
tent by  the  lightness  of  the  lower  valve. 


GASES   AND   LIQUIDS  33 

the  pipe  beneath  the  lower  valve,  which  is  located  at  the 
bottom  of  the  pump-barrel. 

In  a  pump  which  has  not  been  used  for  any  consid- 
erable time,  the  piston  is  likely  to  become  dry  and  to 
shrink  sufficiently  to  permit  air  to  pass  between  it  and 
the  walls  of  the  pump-barrel.  A  small  quantity  of  water 
poured  into  the  barrel  will  remedy  this  defect  tempora- 
rily. Why  ? 

29.  Height  to  Which  Water  may  be  Raised  by  the  Lifting- 
Pump. — In  the  vacuum  fountain  experiment  (§  7),  the 
pressure  of  the  air  within  the  vacuum  fountain  vessel  was 
less  than  the  pressure  of  the  air  on  the  surface  of  the 
water  in  the  jar  into  which  the  open  end  of  the  tube  at 
the  base  of  the  vessel  was  dipped.  In  consequence  of  the 
greater  pressure  of  the  outer  air,  the  water  in  the  jar  was 
forced  up  into  the  vessel.  In  a  similar  manner,  after 
pumping  has  reduced  the  quantity  of  air  in  the  pipe  below 
the  pump-barrel,  and  has  thus  diminished  the  pressure 
of  the  air  which  remains  in  the  pipe,  the  pressure  of  the 
outer  atmosphere  on  the  water  in  the  well  is  able  to  push 
a  part  of  the  water  up  the  pipe. 

The  distance  the  water  is  forced  up  the  pipe  depends 
upon  the  degree  of  exhaustion  of  the  air  in  the  pipe,  and 
upon  the  amount  of  pressure  exerted  by  the  outer  atmos- 
phere at  the  time  of  pumping.  It  has  been  shown  that 
under  the  most  favorable  circumstances,  the  atmosphere 
at  sea  level  can  hold  up  only  about  34  feet  of  water  (§  8). 
This  is,  therefore,  the  extreme  height  to  which  the  atmos- 
phere can  push  water  up  a  pump-stocL  A  rise  of  34  feet 
demands  a  perfect  vacuum  in  the  pipe  up  which  the  water 
is  forced.  This  state  of  exhaustion  is  never  reached  by  a 
water-pump.  In  practice,  the  pump-maker  does  not  ex- 
pect the  water  to  rise  much  higher  than  28  feet.  He, 


34  ELEMENTARY   PHYSICS 

therefore  chooses  a  pump-barrel  of  such  length  that  the 
valve  in  the  bucket  is  not  more  than  28  feet  from  the  sur- 
face of  the  water  in  the  well. 

In  order  to  pump  water  from  a  deep  well  it  is  necessary 
to  construct  a  pump -barrel  of  such  length  that  it  will  ex- 
tend from  the  pump -spout  down  to  within  28  feet  of  the 
surface  of  the  water  in  the  well.  The  bucket  is  fastened 
to  a  rod  of  such  length  that  when  the  bucket  is  on  its 
downward  stroke  it  descends  to  within  one  or  two  inches 
of  the  bottom  of  the  pump-barrel.  In  case  the  pump- 
barrel  is  100  feet  long,  a  vertical  height  of  nearly  100  feet 
of  water  is  lifted  within  the  pump-barrel  at  every  upward 
stroke  of  the  pump-rod,  when  once  the  water  has  begun 
to  flow  out  at  the  spout.  Every  down-stroke  of  the 
pump-handle  causes  an  up -stroke  of  the  pump-rod  and 
bucket. 

30.  Detailed  Description  of  Action  of  Lifting-Pump 

When  the  lifting-pump  is  first  set  in  operation  it  acts  like 
an  air-pump  in  removing  the  air  from  the  pipe  below  the 
pump-barrel.  When  the  piston,  or  bucket,  is  raised  for  the 
first  time,  the  air  in  the  lower  part  of  the  pump-barrel  ex- 
•  pands  so  as  to  fill  the  extra  space  below  the  bucket.  This 
reduces  the  pressure  of  the  air  in  the  barrel  below  that  of 
the  air  in  the  pipe.  The  air  in  the  pipe  is  able,  therefore, 
to  push  up  the  lower  valve,  and  a  part  of  the  air  in  the 
pipe  enters  the  lower  part  of  the  barrel.  Owing  to  the 
diminution  in  the  quantity  of  air  in  the  pipe  situated 
below  the  lower  valve,  there  is  also  a  reduction  of  the 
pressure  of  the  air  remaining  in  the  pipe,  so  that  the  at- 
mosphere presses  more  strongly  upon  the  surface  of  the 
water  in  the  well  than  the  air  within  the  pipe  does  upon 
the  water  immediately  beneath  it.  This  unequal  pressure 
of  air  causes  water  to  rise  in  the  pipe,  until  the  downward 


GASES   AND   LIQUIDS  35 

pressure  of  the  air  remaining-  in  the  pipe  together  with 
the  downward  pressure  of  the  water  which  has  risen 
within  the  pipe  balances  the  pressure  of  the  atmosphere 
on  the  water  in  the  well. 

Each  successive  stroke  of  the  bucket  permits  more  air 
to  escape  from  the  pipe  into  the  barrel.  By  this  means 
the  pressure  of  the  air  remaining-  within  the  pipe  is  re- 
duced more  and  more,  and  this  permits  the  pressure  of 
the  outside  air  on  the  water  in  the  well  to  force  more 
water  into  the  pipe,  so  that  it  will  rise  to  higher  and 
higher  levels  within  the  pipe.  After  each  stroke  the 
water  rises  in  the  pipe  until  the  downward  pressure  of 
the  air  and  of  the  water  which  has  risen  within  the  pipe 
balances  the  pressure  of  the  atmosphere  on  the  water  in 
the  well. 

Finally,  if  the  valve  in  the  bucket  is  not  too  far  above 
the  surface  of  the  water  in  the  well,  some  of  the  water  in 
the  pipe  is  forced  above  the  lower  valve  of  the  pump. 
This  leaves  a  small  quantity  of  air  between  the  bucket  and 
the  surface  of  the  water  which  has  now  risen  above  the 
lower  valve.  At  the  next  descent  of  the  bucket,  in  ad- 
dition to  this  last  remnant  of  air,  a  part  of  the  water  in 
the  barrel  is  also  forced  above  the  valve  in  the  bucket.  At 
the  next  ascent  of  the  bucket  no  air  remains  beneath  it  in 
the  pump-barrel.  Consequently,  the  water  which  is  forced 
up  the  pipe  by  the  outside  air  follows  immediately  after 
the  bucket  and  fills  the  space  below  it. 

At  the  next  descent  of  the  bucket  the  water  at  the  bot- 
tom of  the  pump-barrel  cannot  escape  downward,  but 
is  forced  through  the  valve  in  the  bucket  into  that  part 
of  the  barrel  which  is  above.  With  each  following  up- 
ward stroke  of  the  bucket  more  and  more  water  enters  the 
lower  part  of  the  pump-barrel,  and  with  each  downward 


36  ELEMENTARY   PHYSICS 

stroke  more  water  is  forced  into  that  part  of  the  pump- 
barrel  which  is  above  the  bucket  (Fig.  24,  A),  until  finally 
a  part  of  the  water  runs  out  of  the  spout  near  the  top. 

The  pump  is  now  started.  Each  downward  stroke  of 
the  bucket  causes  some  of  the  water  which  has  last  en- 
tered the  lower  part  of  the  pump -barrel  to  pass  up 
through  the  valve  in  the  bucket,  and  each  upward  stroke 
lifts  the  water  which  is  above  the  bucket  sufficiently  to 
allow  the  upper  part  of  the  water  thus  lifted  above  the 
level  of  the  spout  to  run  out  (Fig.  24,  B). 

31.  The  Force-Pump.— The  force-pump,  in  its  construc- 
tion and  action,  is  quite  similar  to  the  lifting-pump. 
The  outward-opening  valve,  however,  is  not  attached  to 
the  piston,  but  is  placed  somewhere  within  the  tube 
which  leads  laterally  from  the  base  of  the  barrel  and 
forms  an  outlet  for  the  water  which  it  contains.  The 
piston  does  not  lift  water  as  in  the  lifting-pump,  but, 
during  its  downward  stroke,  forces  the  water  out  of  the 
base  of  the  barrel,  through  the  tube  (Fig.  25). 

Neither  form  of  pump  produces  a  steady  stream.  The 
flow  of  water  is  greater  during  the  upstroke  of  the  piston 
in  the  lifting-pump,  and  during  the  down-stroke  in  the 
force-pump. 

The  flow  of  water  from  a  force-pump,  however,  may  be 
made  much  more  steady  by  employing  an  air-chamber. 
This  air-chamber  consists  of  a  hollow  vessel,  open 
only  at  the  base.  It  is  usually  provided  with  two 
openings,  one  for  the  entrance  of  water  coming  from 
the  pump,  the  other  for  the  exit  of  water  into  a  second 
tube.  In  this  case  the  name  delivery  tube  may  be  applied 
to  the  tube  through  which  the  water  escapes  from  the 
air-chamber.  In  the  case  of  a  fire-engine  a  nozzle  is  at- 
tached to  the  farther  end  of  the  delivery  tube  where  the 


GASES   AND   LIQUIDS 


37 


water  escapes.  The  nozzle  of  the  delivery  tube  is  made 
so  narrow  that  the  pump  can  force  water  into  the  air- 
chamber  more  readily  than  the  water  can  escape  through 
the  nozzle.  This  causes  the  water  to  rise  in  the  air-chamber, 
and  to  compress  the  air,  which 
the  water  confines  to  the  upper 
part  of  the  chamber.  As  the 
water  rises  in  the  air-chamber, 
the  air  imprisoned  in  its  upper 
portion  is  more  and  more  com- 
pressed. The  compressed  air  ex- 
erts a  counter-pressure  upon  the 
water  in  the  air-chamber,  and 
forces  it  out  through  the  nozzle 
at  the  end  of  the  delivery  tube. 
This  action  of  the  imprisoned  air 
continues  to  force  out  water 
through  the  nozzle  at  the  end  of 
the  delivery  tube  even  while  the 
piston  is  rising  in  the  pump- 
barrel.  The  valve  in  the  delivery 
tube  is  at  the  base  of  the  air- 
chamber  and  prevents  the  return 
of  the  water  to  the  pump  during 
the  upstroke  of  the  piston. 

If  the  pump  is  worked  with  suf- 
ficient rapidity  the  piston  begins 
to  descend  before  the  air  in  the 
air-chamber  can  force  out  much  of  the  water  in  the  lower 
part  of  the  chamber.  The  more  rapid  the  action  of  the 
pump,  the  shorter  is  the  time  during  which  the  pressure 
of  the  air  in  the  chamber  may  decrease,  and  the  more 
steady  will  be  the  outflow  of  the  water  at  the  end  of  the 


FIG.  25. 


38  ELEMENTARY   PHYSICS 

nozzle.  Moreover,  the  more  rapid  the  action  of  the 
pump,  the  greater  is  the  amount  of  water  forced  up  into 
the  air-chamber,  the  greater  is  the  degree  of  compression 
suffered  by  the  air,  and  the  swifter  is  the  stream  of  water 
forced  out  through  the  nozzle. 

32.  Stop-cocks.— A  stop-cock  is  a  contrivance  by  means 
of  which  the  passage  through  a  tube  can  be  either  partly 
or  entirely  opened  and  closed  at  wilL  The  essential  part 
of  a  stop-cock  consists  in  a  slightly  tapering  stopper  or 
plug,  commonly  known  as  the  spigot,  inserted  air-tight 
into  a  hole  which  passes  at  right  angles  through  the 
walls  of  the  tube.  The  plug  is  fastened  in  such  a  man- 
ner, that,  while  it  cannot  be  withdrawn,  it  can  be  readily 
turned.  A  hole  is  drilled  crosswise  through  the  plug  at 
such  a  point  that,  when  the  plug  is  turned  in  one  posi- 
tion, this  hole  is  in  line  with  the  tube  and  thus  forms  a 
passage  continuous  with  the  tube.  When,  however,  the 
plug  is  turned  either  to  the  right  or  to  the  left,  the  hole 
through  the  plug  is  turned  away  from  the  passage 
through  the  tube,  thereby  preventing  all  movement  of 
liquid  or  gas  farther  than  the  walls  of  the  plug. 

Sometimes  a  stop-cock  is  used  not  merely  to  open  or 
to  close  the  passage  through  a  tube,  but  also,  when  occa- 
sion requires,  to  connect  the  interior  of  the  tube  with 
the  outside.  This  can  be  accomplished  by  drilling  a  hole 
transversely  through  the  plug  or  spigot,  as  in  the  previ- 
ous case,  and  then  drilling  another  hole  at  a  right  angle 
to  the  first,  extending  from  the  surface  on  one  side  of  the 
plug  as  far  as,  but  not  beyond,  the  other  hole  which 
passes  through  the  centre  of  the  plug.  In  other  words, 
the  two  holes  in  the  plug  join  so  as  to  form  the  letter  T. 

When  the  hole  which  passes  entirely  through  the  plug 
is  in  line  with  the  tube,  the  second  hole,  which  joins  it 


GASES   AND   LIQUIDS 


39 


at  right  angles,  points  toward  one  of  the  lateral  walls  of 
the  tube.  An  opening  is  drilled  through  this  wall  of  the 
tube  so  that  in  one  position  of  the  plug  the  first  hole 
through  the  plug  is  in  line  with  the  tube  and  the  second 
hole  is  in  line  with  the  opening  in  the  wall  (Fig.  26,  a). 
In  this  position  a  fluid  may  pass  through  the  plug  out 
into  the  open  air,  or  in 
the  inverse  direction. 
This  is  the  position  of 
some  plugs  used  to  ad- 
mit air  to  the  tube  con- 
necting the  air-pump 
with  the  bell- jar,  when  it 
is  desired  to  permit  air 
to  return  into  a  bell- jar 
from  which  air  has  pre- 
viously been  removed.  If 
the  plug  is  turned  half 
way  around,  the  passage 
through  the  tube  is  un- 
obstructed, but  there  is 
no  connection  with  the 
side  opening  (Fig.  26,  b). 
At  certain  intermediate 
positions  of  the  stop- 
cock the  passage  through  the  tube  is  entirely  obstructed 
(Fig.  26,  c). 

33.  Manometer. — A  manometer  is  an  instrument  for 
determining  the  degree  of  exhaustion  of  the  air  from  a 
bell-jar  or  other  vessel.  It  consists  essentially  of  a  closed 
cylindrical  vessel  containing  a  narrow  U-shaped  tube 
closed  at  one  end.  The  closed  arm  is  filled  with  mercury, 
great  care  being1  taken  to  exclude  all  air  from  this  arm. 


<    -          ~IJ 

*±~r~ 

2-     ,;:    I    £/;?;•;.  •;,",••  " 

FIG.  26. 


40  ELEMENTARY   PHYSICS 

The  mercury  is  prevented  from  running  over  into  the 
open  arm  by  the  pressure  of  the  air  (Fig.  23).  As  soon 
as  the  pressure  of  the  air  in  the  manometer  is  reduced  by 
pumping  so  much  that  it  is  no  longer  sufficient  to  sustain 
the  weight  of  all  the  mercury  in  the  closed  arm,  the 
mercury  in  this  arm  falls  and  a  part  runs  up  into  the 
open  side. 

The  proportion  of  air  still  remaining  may  be  deter- 
mined by  noticing  the  height  to  which  the  upper  surface 
of  the  mercury  in  the  closed  arm  rises  above  that  in  the 
open  arm.  If  the  mercury  in  the  barometer,  at  the  time 
of  the  use  of  the  manometer,  rises  to  a  height  of  30  inches 
and  the  difference  in  the  level  of  the  mercury  in  the  two 
arms  of  the  manometer  tube  is  3  inches,  only  TV  as  much 
pressure  is  exerted  by  the  air  in  the  manometer  as  by  the 
atmosphere  outside.  This  indicates  that  -fa  of  the  air 
has  been  pumped  out  of  the  manometer.  When  all  of 
the  air  has  been  removed,  the  mercury  assumes  the  same 
level  on  both  sides  of  the  manometer  tube.  As  soon  as 
the  air  is  admitted  to  the  vessel,  the  mercury  is  pushed 
back  into  the  closed  arm  of  the  tube.  In  order  to  have 
the  space  under  the  bell-jar  entirely  free  for  experimental 
purposes,  the  manometer  is  usually  attached  to  the  tube 
connecting  the  air-pump  with  the  bell- jar.  The  degree 
of  exhaustion  of  the  air  in  the  bell- jar  is  accompanied 
by  an  equal  degree  of  exhaustion  in  the  manometer  and 
is  determined  as  soon  as  the  degree  of  exhaustion  in 
the  manometer  is  known. 

Since  the  manometer  is  used  chiefly  to  determine  the 
amount  of  air  remaining  in  a  vessel  after  almost  all  of 
the  air  has  been  removed,  it  is  not  necessary  to  use  a 
U-tube  more  than  8  or  10  inches  long.  In  that  case  the 
mercury  will  not  fall  until  the  degree  of  exhaustion  of 


GASES   AND   LIQUIDS  41 

air  in  the  manometer  is  so  great  that  the  air  can  no 
longer  support  the  8  or  10  inches  of  mercury  present  in 
the  closed  arm  at  the  beginning  of  the  experiment. 

34.  The  Tension  of  Liquid  Films. — Dip  a  pipe  into  soap- 
suds and  produce  a  soap-bubble  at  the  bowl  of  the  pipe. 
Notice  that  it  is  necessary  to  blow  with  some  force  to  en- 
large it.  When  the  mouth  is  removed 
from  the  stem  of  the  pipe,  the  film 
contracts  and  the  bubble  becomes 
smaller. 

Wet  a  funnel  thoroughly  with 
water.  Form  a  film  across  the  larger 
end  of  the  funnel  by  dipping  it  into 
soapsuds.  Blow  into  the  funnel  so 
as  to  expand  the  film  (Fig.  27,  A) 

and  then  remove  the  mouth.  The  film  contracts  and  on 
this  account  travels  inward  towards  the  narrow  end  of  the 
funnel  (Fig.  27,  B). 

Make  a  wire  ring  about  4  inches  in  diameter,  and  twist 
the  ends  of  the  wire  so  as  to.  form  a  handle.  Fasten  the 
ends  of  a  short  piece  of  thread  to  points  on  the  wire  ring 
about  3  inches  apart,  so  that  the  thread  will  hang  down 
loosely  for  a  short  distance.  Dip  the  ring  into  soap- 
suds. A  film  forms  across  the  entire  ring  and  supports 
the  thread  (Fig.  28,  A).  By  means  of  a  pencil  the  thread 
can  be  moved  from  side  to  side  without  destroying  the 
film.  Break  the  film  in  the  narrower  of  the  two  spaces 
between  the  ring  and  the  thread.  All  of  the  film  within 
this  space  at  once  disappears.  The  film  within  the 
larger  space  contracts  and  stretches  the  thread,  so  that 
the  latter  forms  a  regular  curve  (Fig.  28,  B). 

Dip  the  ring  a  second  time  into  the  soapsuds  and 
pierce  the  film  in  the  broader  space  between  the  ring  and 


42  ELEMENTARY   PHYSIOS 

the  thread.  That  part  of  the  film  within  the  narrower 
space  draws  the  thread  against  the  wire  ring.  Catch 
hold  of  the  thread  and  pull  it  toward  the  central  part  of 

the  ring.  The  film  is 
re-formed.  Let  go. 
The  thread  is  pulled 
back  as  though  the 
film  consisted  of 
stretched  rubber. 

It  is  evident  in  all 
of  these  experiments 
that   thin    films    of 
liquids  act  more  or 
FIG.  28.  less    like    stretched 

membranes. 

The  experiments  will  be  more  likely  to  succeed  if  a 
small  quantity  of  glycerine  is  added  to  the  soapsuds. 

35.  Surface  Tension  of  Liquids. — Fill  a  test-tube  with 
water.  Keep  the  edges  of  the  test-tube  dry  and  add 
water  drop  by  drop.  The- level  of  the  water  will  rise 
considerably  above  the  edges  of  the  tube  without  over- 
flowing. It  behaves  as  though  it  were  covered  by  a  thin 
film  attached  to  the  edges  of  the  glass. 

Dip  a  narrow  glass  tube  into  water  and  then  take  it  out. 
A  small  part  of  the  water  which  rose  into  the  tube  still 
remains  in  the  tube.  The  lower  part  of  the  water  sags 
below  the  level  of  the  glass  and  behaves  very  much  as  if 
it  were  held  up  by  a  sac  or  thin  film  attached  to  the 
margin  of  the  glass. 

Hold  a  small  needle  horizontally  within  a  short  dis- 
tance of  the  water  in  a  vessel.  Drop  it.  It  floats  on  the 
surface,  but  the  part  of  the  surface  nearest  the  needle  is 
now  depressed  and  forms  a  little  trough  at  the  bottom  of 


GASES   AND   LIQUIDS  43 

which  rests  the  needle  (Fig.  29).    Press  on  the  needle  and 

see  how  quickly,   after  it  has  once  broken  through  the 

surface,  it  falls  to  the  bottom  of  the  vessel.     If  the  needle 

is   removed   without    breaking 

through  the  surface,  the  surface 

flattens  again.     The  surface  of 

the  water  in  the  vessel  behaves  as  though  it  consisted  of 

a  stretched  film  which  is  depressed  by  the  weight,  of  the 

needle  but  which  springs  back  as  soon  as  the  needle  is 

removed. 

This  imaginary  surface  film  appears  to  have  considera- 
ble strength  and  can  be  subjected  to  considerable  tension 
before  breaking.  If,  for  instance,  an  aluminum  medal 
about  as  large  as  a  silver  dollar  be  carefully  placed  in  a 
horizontal  position  on  the  surface  of  the  water,  the  medal 
will  float,  although  it  forms  a  depression  in  the  surface 
of  the  water. 

The  individual  hairs  of  a  camel's-hair  brush  when  dry 
stand  slightly  apart,  producing  a  somewhat  bushy  ap- 
pearance. Dip  the  brush  into  water.  The  bushy  appear- 
ance remains  while  the  brush  is  in  the  water.  The  mere 
presence  of  water  is  not  sufficient  to  alter  its  appearance. 
Take  the  brush  out  of  the  water.  The  surface  of  the 
water  adhering  to  the  brush  at  once  contracts  and  draws 
the  individual  hairs  together  again,  giving  the  brush  a 
pointed  appearance.  The  surface  of  a  liquid  acts  as  if 
it  were  covered  by  a  film  always  tending  to  contract. 

The  surface  of  a  small  quantity  of  mercury  contracts 
until  it  can  contract  no  longer.  It  then  forms  a  spher- 
ical mass  and  may  be  called  a  drop  of  mercury.  A  drop 
of  water  is  merely  a  small  quantity  of  water  whose  sur- 
face has  contracted  so  much  that  a  spherical  form  is 
assumed.  Melted  lead  poured  through  a  sieve  at  the 


44  ]     EMENTARY   PHYSICS 

top  of  a  high  t*s-.tjer  appears  at  the  bottom  as  rounded 
shot. 

On  account  of  phenomena  similar  to  those  here  de- 
scribed, it  is  convenient  to  refer  to  the  surface  of  a  liquid 
as  if  it  were  covered  by  a  surface  film,  and,  since  this 
film  appears  to  be  stretched  or  in  a  state  of  tension,  the 
expression  surface  tension  is  used  in  the  discussion  of 
many  phenomena.  An  explanation  of  the  cause  of  the 
phenomena  known  as  surface  tension  requires  the  use  of 
the  molecular  theory.  After  this  theory  has  been  taken 
up,  it  is  well  to  return  to  a  consideration  of  these  phe- 
nomena. Briefly  stated,  the  molecules  within  the  liquid 
are  equally  attracted  in  all  directions  by  surrounding1 
molecules,  and  hence  do  not  give  any  evidence  of  tension. 
However,  those  at  the  surface  are  attracted  only  down- 
ward and  laterally,  not  upward.  Hence  the  surface  mole- 
cules act  as  though  they  formed  a  laterally  stretched 
membrane  held  tight  against  the  main  body  of  water. 

36.  Glassworking.—  To  cut  a  glass  tube,  lay  it  on  the 
table,  and,  by  means  of  a  single  forward  stroke  with  a 
three-cornered  file,  make  a  short  but  deep  scratch  across 
the  tube.  Placing  the  thumbs  directly  behind  the  scratch, 
gently  push  with  the  thumbs  against  the  tube,  at  the 
same  time  pull  with  the  hands,  and  the  tube  will  break  at 


the  tube  is  large,  wrap  it  in 
cloth  before  trying  to  break  it. 
Rotate  the  end  of  the  tube 
slowly  in  the  flame  of  a  Bunsen 
burner  until  the  flame  turns 
yellow.  The  glass  is  softened 

by  the  heat  and  its  surface  is  melted.     In  consequence 
of  the  shrinking  of  the   surface   film   of  the  glass  the 


GASES   AND    LIQUID' 


45 


FIG.  31. 


sharp  edges  at  the  end  of  the  tube  arc;  Bounded.  If  the 
end  of  the  tube  is  held  a  longer  time  :'  i  the  flame,  the 
j  contraction  of  the 

surface  film  begins 
to  diminish  the  size 
of  the  opening,  and 
this  may  be  contin- 
ued until  an  opening 
of  any  desired  size 
is  obtained,  or  until 
the  opening  entirely 
closes.  "When  the  tube  has  melted  shut  for  a  distance 
back  from  the  end  exceeding  the  thickness  of  the  walls 
of  the  tube,  blow  quickly  but  evenly  into  the  open  end. 
The  surface  film  resists  the  expansion  of  the  glass  in  all 
directions  so  evenly  that  the  glass  takes  the  form  of  a 
small  bulb  or  sphere.  If  the  melted  glass  at  the  end  of 
the  tube  is  sufficiently  soft,  a  sphere  may  be  blown  as 
large  as  a  soap-bubble. 

To  bend  a  glass  tube  heat  evenly  a  length  of  two 
inches,  moving  the  tube  to  right  and  left  and  rotating  it 
in  the  luminous  part  of  an  ordinary  gas  flame  (Fig.  31). 
As  soon  as  the  glass  becomes  soft  enough,  bend  it  a 
little  at  every  point  along  the  heated  portion,  but  avoid 
any  sharp  bend  (Fig.  32). 
If  a  length  of  two  inches 
is  heated  long  enough  to 
be  soft,  and  the  ends  of 
the  tube  are  gently  drawn 
apart,  the  glass  will  stretch 
like  taffy,  and  the  diam- 
eter of  the  tube  will  become  narrower  at  this  point. 
If  considerably  heated  and  rapidly  drawn  apart,  the 


FIG.  32. 


46  ELEMENTARY   PHYSICS 

stretched  part  of  the  tube  may  become  as  fine  as  a  coarse 
hair. 

In  order  to  secure  a  glass  handle  for  a  platinum  wire 
(§  229),  heat  a  small  part  of  a  glass  tube  and  stretch  it 
moderately  so  that  the  diameter  of  the  heated  part  is 
reduced  about  one  half  (Fig.  31).  If  the  heated  part  is 
moderately  stretched,  the  walls  of  the  tube  remain  thicker. 

A  Cut  this  part  with 

|l  a  file  as  above  de- 

scribed. Insert  the 
platinum  wire  in  this 
end  of  the  glass  tube 

and  place  it  in  the  flame  of  the  Bunsen  burner  (Fig.  33). 
The  glass  will  close  in  around  the  wire  and  hold  it  in 
position. 

In  order  to  secure  a  platinum  tip  at  the  end  of  the  de- 
livery tube  for  experiments  with  burning  hydrogen  (§  149), 
roll  a  piece  of  platinum  foil  around  that  end  of  a  file 
which  enters  the  handle.  Withdraw  the  hollow  conical 
tube  thus  formed  and  slip  it  into  a  glass  tube  (which  has 
been  stretched  and  cut  in  the  manner  described  above) 
so  that  the  pointed  end  of  the  platinum  tube  projects. 
Heat  the  narrowed  end  of  the  glass  tube  until  it  closes 
in  around  the  larger  end  of  the  platinum  tube. 

The  opening  of  a  glass  tube  may  be  enlarged  while  it 
is  heated  by  spreading  it  with  the  narrow  end  of  a  file. 

37.  Attraction  of  Water  for  Water.— The  force  with 
which  the  different  parts  of  a  mass  of  water  hold  together 
can  be  shown  by  the  following  experiment. 

Secure  a  circular  piece  of  ordinary  window-glass, 
4  inches  in  diameter.  By  means  of  sealing-wax,  attach 
three  strings  at  equidistant  points  near  the  margin  of 
the  disk.  Tie  the  ends  together  and  fasten  them  to  the 


GASES  AND   LIQUIDS 


47 


hook  beneath  a  delicate  spring  balance  (8-ounce,  gradu- 
ated to  J-oiince  intervals)  in  such  a  manner  that  the  disk 
will  hang  perfectly  horizontal.  Determine  the  weight  of 
the  disk.  Place  a  vessel  of  water  beneath  the  glass  disk, 
and  lower  the  balance  until  the  glass  disk  comes  in  con- 
tact with  the  water. 

Slowly  lift  the  balance.  The  disk  gradually  rises  and 
pulls  a  small  part  of  the  water  above  the  general  level  of 
the  water  in  the  vessel  (Fig.  34).  As 
the  balance  is  lifted  higher  and  high- 
er, the  index  of  the  balance  is  pulled 
lower  and  lower,  showing  that  the 
water  in  the  vessel  is  pulling  down 
the  disk.  At  the  same  time  the 
amount  of  water  pulled  by  the  disk 
above  the  general  water  level  in- 
creases. Finally  the  disk  flies  up- 
ward. It  seems  to  have  been  pulled 
away  from  the  water  in  the  vessel. 
On  examining  the  lower  surface  of 
the  disk,  however,  a  small  quantity 
of  water  is  seen  clinging  to  its  lower 
surface.  This  shows  that  the  sepa- 
ration did  not  occur  between  the  disk 
and  the  water,  but  that  the  film  of 
water  still  clinging  to  the  disk  was 
pulled  away  from  the  main  body  of 
water  remaining  in  the  vessel.  The 
film  of  water  does  not  remain  spread  evenly  over  the  sur- 
face of  the  disk,  but  soon  collects  together  in  little  drops, 
(§  35). 

The  balance  indicates  what  force  is  required  to  pull  the 
water  attached  to  the  disk  away  from  the  water  in  the  vessel. 


FIG.  34. 


ELEMENTARY   PHYSICS 


38.  Adhesion  and  Cohesion. — It  will  be  noticed  that  a 
film  of  water  separates  from  the  water  remaining  in  the 
vessel  and  clings  to  the  glass  plate.     This  indicates  that 
the  attraction   of  water  for  glass  is  greater  than  the 
attraction  of  water  for  water.     The  attraction   between 
unlike  substances,  for  instance  between  water  and  glass, 
is  called  adhesion.     The   attraction  between  masses  of 
the  same  kind  of  substances,  for  example  the  attraction 
of  the  water  film  for  the  water  in  the  vessel,  is  called  co- 
hesion.    The  adhesion  between  water  and  glass  is  greater 
than  the  cohesion  between  water  and  water. 

39.  Water  Drawn  Up  on  Vertical  Walls  of  Glass.— The 
attraction  of  water  for  glass  can  be  shown  by  other  phe- 
nomena.    A  phenomenon  is  not  necessarily  an  unusual  or 
striking  occurrence.     Indeed,  most  phenomena  are  quite 
ordinary  or  commonplace.    Any  change  of  position,  form, 
color,  or  chemical  composition  which  takes  place,  in  fact, 
anything  which  happens,  whether  caused  by  man  or  by 
natural  forces,  is  often  called  a  phenomenon  in  physics. 

Examine  the  water  which  has  been  placed  in  a  tumbler. 
The  surface  of  the  water  is  flat   except  at  the  edges, 
where  it  curves  upward  against  the  sides 
of  the  glass. 

Dip  the  lower  part  of  a  plate  of  glass 
vertically  into  water.     Notice  how  high 
the  water  rises  on  the  sides  of  the  glass 
(Fig.  35).    Now  dip  two  plates  into  water 
so  that  they  form  a  very  small  angle  with 
one  another,  the  two  vertical  edges  on 
one  side  being  in  contact.     The  water 
rises  to  a  greater  height   between  the 
plates  than  it  does  on  their  outer  surface,  especially  where 
the  distance  between  the  plates  is  very  small  (Fig.  36). 


GASES   AND   LIQUIDS 


49 


FIG.  36. 


In  all  of  these  experiments  the 
attraction  between  glass  and 
water  causes  the  water  to  creep 
up  along  the  sides  of  the  glass. 
In  the  experiment  last  described, 
the  water  rises  to  the  greatest 
height  where  the  two  plates  of 
glass  come  in  contact  with  each 
other,  for  at  this  point  both  plates 
act  at  the  same  time,  and  from 
within  a  very  short  distance,  upon  the  same  particles  of 
water. 

40.  Capillary  Tubes,— Take  glass  tubes  of  different  sizes, 
the  internal  diameter  or  bore  of  the  largest  tube  not  ex- 
ceeding -J  of  an  inch.  Arrange  them  in  a  series,  beginning 
with  the  tube  of  the  largest  bore.  Fasten  the  tubes  ver- 
tically in  small  openings  bored  through  a  short  piece  of 
_  wood.  The  lower  ends  of  the 
tubes  should  be  at  the  same 
level.  Dip  the  tubes  vertically 
into  water.  Notice  in  the  vari- 
ous tubes  the  relation  between 
the  size  of  the  bore  and  the 
height  to  which  the  water 
rises,  and  the  degree  and  direc- 
tion of  curvature  of  the  upper 
surface  of  the  water  within 
the  tube  (§  43). 

Water  rises  to  the  greatest 
height  in  the  tube  having  the 
smallest  bore  (Fig.  37).     The 
rule  is  that  The  vertical  rise  of 
FIG.  37.  the  water  varies  inversely  as  the 


50 


ELEMENTARY   PHYSICS 


-  Gas  caused 
by  vaporization 
of  the  liquid 
wing  to  the  heat. 


diameter  of  the  tube.  In  a  tube  having-  one-half  the  diam- 
eter water  will  rise  to  twice  the  height.  This  principle  is 
shown  in  the  most  striking-  manner  by  tubes  whose  inter- 
nal diameter  is  exceedingly  small.  Tubes  of  glass  have 
been  constructed  whose  internal  diameter  does  not  ex- 
ceed that  of  a  hair,  and,  since  tubes  of  this  size  were  often 
used  in  experiments  of  this  nature,  the  action  of  liquids  in 

narrow  tubes  is  often  re- 
ferred to  as  the  action  of 
liquids  in  hair-like  or  capil- 
lary tubes,  or,  as  capillarity. 
The  ascent  of  liquids  in  fine 
capillary  tubes  is  well  illus- 
trated in  the  ascent  of  oil  or 
alcohol  along  the  wick  of  a 
lamp.  On  approaching  the 
flame,  the  oil  or  alcohol  va- 
porizes or  turns  into  gas. 
Where  this  gas  comes  in 
contact  with  the  air  it  burns, 
when  once  lighted.  Close 
to  the  wick  the  gas  is  given 
off  too  rapidly  to  become 
mingled  with  air  and  here 
there  is  no  flame  (Fig.  38). 

Dip  tubes  of  different  di- 
ameters into  mercury  (Fig.  39),  and  notice  the  relation- 
ship existing  between  the  size  of  the  bore,  the  amount  of 
depression  of  the  mercury  in  the  tubes  below  the  general 
level  of  the  mercury  in  the  vessel,  and  the  degree  and 
direction  of  curvature  of  the  surface  (§  43).  What  is  the 
difference  in  the  direction  of  the  curvature  of  the  surface 
of  the  water  and  of  the  mercury  in  these  tubes  ? 


PIG.  38. 


GASES  AND   LIQUIDS 


51 


41.  Concave    and    Convex    Surfaces.—  Kegularly  curved 
surfaces  are  distinguished  either  as  concave  or  convex.    If 
an  observer,  on  looking"  at  a 

regularly  curved  surface,  finds 
that  the  edges  curve  toward 
him,  so  that  from  his  point  of 
view  the  surface  seems  to  be 
hollowed  out,  the  surface  is 
said  to  be  concave  (Fig.  40). 
When,  however,  the  surface 
from  the  centre  to  the  margin 
curves  away  from  him,  so  as 
to  have  the  appearance  of  a 
part  of  a  solid  globe  or  sphere, 
the  surface  is  said  to  be  con- 
vex. When  viewed  from  the 
side,  regularly  curved  surfaces 
are  said  to  be  concave  when 
the  edges  curve  upward  and  convex  when  the  edges  are 
curved  downward. 

Which  of  the  surfaces  in  the  experiments  above  are 
concave  and  which  are  convex  ? 

42.  Comparison  of  Cohesion  of  Water  and  of  Cohesion  of 

Mercury  with  the  Ad- 
h  e  s  i  o  n  of  These  Sub- 
stances to  Glass. — It  has 
been  explained  that 
water  creeps  up  on  the 
sides  of  a  piece  of  glass 
because  the  attraction 
of  water  for  water  is 
less  than  the  attraction  of  water  for  glass.  This  accounts 
for  the  concave  surface  of  the  water  in  the  glass  tubes. 


FIG.  39. 


Concave 


FIG.  40. 


52  ELEMENTARY   PHYSICS 

When  a  small  glass  plate  is  dipped  in  mercury,  the 
mercury  seems  to  shrink  from  contact  with  the  glass 
(Fig.  41).  The  attraction  of  mercury  for 
mercury  is  greater  than  the  attraction  of 
mercury  for  glass.  In  consequence,  the 
main  body  of  the  mercury  within  the 
vessel  draws  that  part  of  the  mercury 
which  is  in  contact  with  the  glass  tow- 
ards itself  so  effectively,  that,  where  the 
upper  surface  of  the  mercury  meets  the 
glass,  the  mercury  is  drawn  away  from 
the  glass  and  in  consequence  its  margin 
curves  strongly  downward.  This  has  the  effect  of  making 
the  upper  surface  of  the  mercury  convex  when  placed 
within  a  narrow  glass  tube. 

If  the  attraction  of  mercury  for  glass  were  as  great  as 
the  attraction  of  mercury  for  mercury,  the  surface  would 
be  perfectly  horizontal,  even  at  its  actual  contact  with 
the  glass. 

43.  Capillary  Action  Due  to  Surface  Tension. — The  sur- 
face of  a  liquid  acts  as  if  it  consisted  of  a  stretched  film 
which  seeks  to  contract  or  shrink  (§  35). 

If  a  plate  of  glass  previously  moistened  is  dipped  in 
water,  the  film  of  water  adhering  to  the  glass  meets  the 
film  forming  the  surface  of  the  water  at  a  right  angle. 
But  these  films  tend  to  contract  and  so  the  corner  at 
which  they  meet  is  rounded  off,  and  the  water  is  said  to 
rise  along  the  glass.  The  contraction  of  the  film  con- 
tinues until  the  weight  of  the  liquid  raised  by  the  film 
above  the  general  level  of  the  water  in  the  vessel  bal- 
ances the  force  with  which  the  film  is  able  to  contract. 

Moisten  a  glass  tube  of  small  bore  so  that  the  interior 
is  covered  by  a  film  of  water.  Dip  the  tube  into  a  vessel 


GASES   AND   LIQUIDS  53 

of  water.  The  film  of  water  lining-  the  interior  of  the 
tube  meets  the  film  forming-  the  surface  of  the  water 
within  the  tube  at  first  at  a  right  angle,  but  the  con- 
traction of  these  films  causes  the  angle  of  contact  to  be- 
come rounded.  Consequently  the  water  rises  along-  the 
walls  of  the  tube.  As  the  contraction  of  the  films  con- 
tinues, the  water  rises  also  at  the  central  portion  of  the 
tube.  Any  further  contraction  of  the  films  causes  the 
water  to  rise  both  along-  the  walls  and  in  the  middle 
regions  of  the  tube. 

When  the  tube  has  not  been  previously  moistened,  a 
thin  film  of  water  creeps  up  the  walls  of  the  tube  owing 
to  the  strong  attraction  of  glass  for  water  (§'  38).  The 
contraction  of  this  film  where  it  comes  in  contact  with 
the  water  in  sthe  tube  causes  the  same  phenomena  as 
those  already  described. 

When  a  narrow  glass  tube  is  dipped  into  mercury,  the 
mercury  does  not  form  a  film  on  the  glass.  Conse- 
quently any  contraction  of  the  surface  of  the  mercury 
within  the  tube  is  not  accompanied  by  a  rise  of  mercury 
along  the  walls.  On  the  contrary,  the  edges  of  the  surface 
of  the  mercury,  where  they  come  in  contact  with  the  glass, 
are  drawn  away  from  the  glass,  since  the  attraction  of  mer- 
cury for  mercury  is  greater  than  the  attraction  of  mer- 
cury for  glass.  This  gives  the  surface  of  the  mercury 
within  the  tube  a  convex  curvature,  and  further  contrac- 
tion of  this  surface  must  be  accompanied  by  a  depression 
of  the  mercury  within  the  tube  below  the  general  level  of 
the  mercury  in  the  vessel.  This  depression  of  the  mer- 
cury in  the  tube  continues  until  the  force  with  which  the 
surface  film  of  the  mercury  within  the  tube  contracts  is 
balanced  by  the  upward  pressure  caused  by  that  part  of 
the  mercury  in  the  vessel  which  rises  above  the  level 


54 


ELEMENTARY    PHYSICS 


of  the  mercury  which  has  been  depressed  within  the 
tube. 

44.  Siphon. — Dip  a  narrow  glass  tube  under  water. 
Place  the  finger  over  the  upper  end,  and,  holding  it  in  a 
vertical  position,-  remove  the  tube.  The  water  is  held  in 
by  the  pressure  of  the  air  against  the  surface  film  of  the 
water  at  the  lower  end  of  the  tube.  Remove  the  finger. 
The  water  drops  out.  Explain. 

Take  a  narrow  glass  tube  about  15  inches  long,  with 
an  internal  diameter  not  exceeding  J-inch,  and  bend 
it  at  5  inches  from  each  end,  so  as  to  form  a  rectangular 
U-shaped  tube.  Fill  the  tube  with  water.  Be  careful  to 
permit  no  air-bubbles  to  remain.  Cover  one  of  the  open- 
ings with  a  finger  and  invert  the  tube,  keeping  the  two 
openings  at  the  same  level.  The  water  is  held  in  at  the 
open  end  of  the  tube  by  the  pressure  of  the  air  against 
the  surface-film  of  water.  Now  remove  the  finger  with- 
out changing  the  level  of  the  tube.  The  water  is  held  in 
at  both  ends  by  the  pressure  of  the  air  (Fig.  42,  A).  The 


FIG.  42 


success  of  this  experiment  depends  upon  the  ends  of  this 
tube  being  held  at  precisely  the  same  level.  Any  slight 
change  in  the  relative  level  of  the  openings  at  once 


GASES   AND   LIQUIDS  55 

causes  the  water  to  escape  from  the  lower  opening 
(Fig.  42,  B). 

Bend  a  glass  tube,  about  36  inches  long,  into  a  rec- 
tangular U-shaped  form,  so 'that  one  arm  will  be  5  inches 
and  the  other  25  inches  long.  Fill  with  water  in  such  a 
way  that  no  air-bubbles  remain,  and,  placing  a  finger 
over  one  of  the  open  ends,  invert  the  tube.  The  pressure 
of  the  air  prevents  the  escape  of  the  water.  Now  remove 
the  finger  and  the  water  flows  out  rapidly.  Try  the  ex- 
periment again,  closing  the  other  arm  of  the  tube.  From 
which  arm  does  the  water  escape  in  both  cases  ?  A  tube 
of  this  form  will  serve  very  well  as  a  siphon. 

45.  Principle  of  the  Siphon.— The  fact  that  the  water 
flows  from  the  shorter  out  through  the  longer  arm,  does 
not  mean  that  the  pressure  of  the  air  is  greater  at  the  end 
of  the  shorter  than  at  the  end  of  the  longer  arm.  In  fact, 
the  pressure  at  the  end  of  the  shorter  arm  is  slightly  less 
than  the  pressure  at  the  end  of  the  other  arm ;  but  so 
slightly  less  that  the  two  pressures  are  practically  equal. 

While  the  pressures  of  the  air  at  the  ends  of  the  two 
arms  are  practically  equal  to  one  another,  that  part 
of  the  air  pressure  which  is  available  for  moving  the 
water  from  either  opening  through  the  tube  to  the  other 
opening,  is  not  the  same  at  the  lower  end  of  the  short  arm 
as  at  the  lower  end  of  the  long  arm  of  the  tube.  A  part 
of  the  pressure  of  the  air  at  the  end  of  the  shorter  arm  is 
used  in  supporting  the  water  in  this  arm,  and  only  the 
remainder  of  the  pressure  can  be  used  to  push  the  water 
toward  the  other  end  of  the  tube.  In  the  same  manner, 
a  part  of  the  air  pressure  at  the  end  of  the  longer  arm  is 
used  to  support  the  water  in  that  arm.  But  this  column 
of  water  is  longer  than  that  in  the  other  arm,  and  hence  a 
smaller  part  of  the  air  pressure  remains  available  foi 


56 


ELEMENTARY   PHYSICS 


pushing  the  water  from  this  side  over  toward  the  shorter 
arm.  Hence,  in  spite  of  the  tendency  of  the  air  to  hold 
up  the  water  in  both  arms,  the  water  is  pushed  from  the 
side  of  the  short  arm  toward  the  long  one. 

The  greater  the  difference  in  the  length  of  the  two 
arms,  the  greater  will  be  the  inequality  of  the  weights 
of  water  to  be  supported,  the  greater  also  will  be  the 
difference  in  the  amount  of  air  pressure  available  for 
pushing  the  water  from  one  side  of  the  tube  toward 
the  other,  and  the  greater  will  be  the  rapidity  of  the 
flow  of  water  out  of  the  longer  arm. 

46.  Use  of  Siphon. — If,  before  removing  the  finger  from 
either  end  of  the  siphon,  the  end  of  the  short  arm  be 
dipped  under  the  surface  of  the 
water  in  a  jar,  and  the  finger  be  then 
removed,  the  water  will  flow  from 
the  longer  arm  as  before  (Fig.  43). 
As  the  water  flows  out  of  the  longer 
arm  it  tends  to  leave  behind  it  an 
empty  space  or  vacuum.  The  press- 
ure of  the  air  on  the  water  in  the  jar 
then  forces  more  water  up  through 
the  shorter  arm,  keeps  both  arms 
full  of  water,  and  thus  makes  possi- 
ble a  constant  flow  of  water  through 
the  siphon. 

In  the  same  manner  it  is  possible  to  remove  liquids 
from  barrels  and  from  carboys  which  are  too  heavy  to  be 
easily  tilted,  but  which  permit  the  insertion  of  a  siphon 
at  the  top.  Siphons  are  often  improvised  from  rubber 
tubes  for  the  purpose  of  emptying  barrels  of  cider. 
Glass  siphons  are  especially  constructed  for  the  purpose 
of  removing  acids  from  large  glass  bottles  called  carboys. 


Fro.  43. 


GASES   AND   LIQUIDS 


57 


FIG, 


The  length  of  any  arm  of  a  siphon  can  be  determined 
by  measuring-  the  vertical  distance  from  the  open  end  of 
the  tube  to  the  highest  point  in 
the  siphon  (Fig.  44).  When  the 
lower  part  of  the  arm  is  dipped 
into  water,  the  length  of  the  arm 
is  found  by  measuring  the  vertical 
distance  from  the  water  surface  to 
the  highest  point  in  the  siphon. 

47.  Downward  Pressure  of  Liq- 
uids on  the  Base  of  Vessels  Having 
Vertical  Sides.— Fasten  a  glass 
cylinder,  whose  lower  end  has 
been  ground  flat,  in  a  vertical 
position  over  a  vessel.  Attach  a  string  to  the  hook  at 
the  centre  of  a  round  brass  plate  whose  upper  surface  is 
perfectly  flat,  and  whose  diameter  slightly  exceeds  the 
diameter  of  the  cylinder.  Draw  the  loose  end  of  the 
string  up  through  the  cylinder  and  fasten  it  to  a  second 
hook  attached  to  the  lower  side  of  one  of  the  pans  of  a 
balance.  At  first  arrange  the  apparatus  so  as  to  allow 
neither  the  string  nor  the  brass  plate  to  touch  the  cylin- 
der. Place  weights  on  the  other  pan  of  the  balance  until 
the  beam  is  horizontal.  Then  move  the  balance  until  the 
string  is  in  the  centre  of  the  cylinder  and  raise  the  bal- 
ance l>eam  until  the  brass  plate  barely  touches  the  base 
of  the  glass  cylinder.  If  the  upper  surface  of  the  brass 
plate  is  perfectly  horizontal,  it  will  now  fit  neatly  against 
the  base  of  the  cylinder.  Pour  water  into  the  cylinder. 
The  water  at  once  escapes  at  the  base. 

Place  an  additional  weight  of  2  ounces  on  the  other 
pan  of  the  balance.  The  brass  plate  is  now  held  up 
against  the  base  of  the  cylinder  by  a  force  of  2  ounces. 


58 


ELEMENTARY   PHYSICS 


Therefore,  it  now  requires  a  force  greater  than  2  ounces 
to  pull  or  push  the  plate  away  from  the  cylinder.  Pour 
a  small  quantity  of  water  gently  into  the  cylinder.  It  re- 
mains in  the  cylinder.  Add  more  water.  When  a  certain 
level  is  reached  the  water  (Fig.  45)  begins  to  run  out  in 
drops  between  the  base  of  the  cylinder  and  the  plate.  If 


FIG.  45. 

the  water  is  poured  into  the  cylinder  too  rapidly,  it  runs 
out  at  the  bottom  in  a  steady  stream. 

Kepeat  the  experiment.  In  each  case  water  begins  to 
flow  out  when  it  has  attained  a  certain  level  within  the 
cylinder.  Measure  the  depth  of  the  water  in  the  cylinder 
at  the  moment  when  the  water  begins  to  escape.  Then 
let  this  water  flow  into  a  small  vessel  and  determine  its 
weight.  It  will  weigh  2  ounces.  This  indicates  that  the 
moment  before  the  water  began  to  escape,  the  weight  in 


GASES   AND   LIQUIDS  59 

the  pan  and  the  downward  pressure  of  the  water  in  the 
cylinder  exactly  balanced.  Any  further  addition  of  water 
to  the  cylinder  causes  the  escape  of  water  at  its  base. 
This  is  in  accordance  with  the  statement  made  in  the 
earlier  part  of  the  paragraph,  that  a  force  of  more  than 
2  ounces  is  required  to  push  the  brass  plate  away  from 
the  cylinder. 

Now  add  2  ounces  to  the  weights  already  present  on 
the  other  pan.  Again  pour  water  into  the  cylinder,  and 
determine  the  depth  of  the  water  in  the  cylinder  when 
the  water  begins  to  escape  at  the  base.  This  depth  is 
found  to  be  twice  that  observed  in  the  preceding  part  of 
the  experiment.  Determine  the  weight  of  the  water  in 
the  cylinder  as  before. 

Eepeat  the  experiment,  using  greater  weights. 

The  observations  so  far  made  are  numerous  enough  to 
justify  the  following  conclusions.  The  downward  pressure 
of  water  on  the  base  of  a  cylinder  varies  with  the  depth  of  the 
water.  Since  the  cylinder  was  placed  in  a  vertical  posi- 
tion, all  the  weight  of  the  water  rests  upon  the  base. 
Therefore,  in  the  case  of  a  vessel  with  a  vertical  wall,  the 
downward  pressure  of  a  liquid  on  the  base  is  equal  to  the 
weight  of  the  liquid  within  the  vessel. 

The  downward  pressure  exerted  by  a  liquid  on  the  base  of 
a  vessel  varies  also  with  the  density  of  the  liquid.  •  To  show 
this  repeat  the  experiment,  using  first  a  saturated  solu- 
tion of  salt  water  and  then  a  lighter  liquid  like  coal  oil. 
With  cylinders  of  different  diameter  it  may  be  shown 
that  the  downward  pressure  exerted  by  a  liquid  varies  with 
the  size  or  area  of  the  surface  which  forms  the  base. 

48.  Downward  Pressure  of  Liquids  on  the  Base  of  Vessels 
Whose  Sides  are  Not  Vertical  or  are  Partly  Vertical.— In- 
stead of  a  glass  cylinder  take  a  vessel  having  the  form  of 


60  ELEMENTARY   PHYSIOS 

a  large  glass  funnel.  The  opening  at  the  lower  and  nar- 
rower end  of  the  funnel  should  be  exactly  equal  in  size  to 
the  opening  at  the  base  of  the  cylinder  used  in  the  pre- 
ceding experiment.  The  base  of  the  funnel  must  be 
perfectly  flat.  Fasten  the  funnel  in  a  vertical  position 
and  arrange  the  brass  plate,  the  balance,  and  the  weights, 
as  at  the  beginning  of  the  preceding  experiment.  In  the 
apparatus  figured,  a  different  device  is  used  which  serves 
the  same  purpose  as  that  described  (Fig.  45). 

Place,  in  succession,  weights  of  2,  4,  6,  or  more  ounces 
upon  the  free  pan  of  the  balance.  After  each  addition  to 
the  weights,  pour  water  into  the  funnel  and  record  the 
height  at  which  the  surface  of  the  water  stands  above  the 
brass  plate  when  the  water  begins  to  escape  at  the  base. 
Compare  these  results  with  the  results  in  the  preceding 
experiment.  It  is  found  that  the  same  depth  of  water  is 
required  to  produce  the  same  pressure  on  the  brass  plate 
at  the  base  of  the  funnel  as  at  the  base  of  the  cylinder. 
This  is  true,  notwithstanding  the  fact  that,  in  order  to 
secure  the  same  depth  of  water  in  the  funnel  as  in  the 
cylinder,  a  much  greater  volume  of  water  is  required. 
From  this  we  may  draw  the  conclusion  that  in  the  case 
of  vessels  with  vertical  or  with  spreading  sides,  the  press- 
ure of  a  liquid  on  the  base  varies  ivith  the  depth  of  the 
water,  but  is  independent  of  the  volume  of  water  used. 

Replace  the  funnel  with  a  vessel  having  a  form  similar 
to  that  of  an  argand  lamp  chimney  (Fig.  45) — narrow 
throughout  the  greater  part  of  its  length,  but  widening 
suddenly  a  short  distance  above  its  base.  The  base 
should  be  ground  flat  and  its  diameter  should  equal  the 
diameter  of  the  cylinder  used  in  the  first  of  this  series 
of  experiments.  Adjust  the  brass  plate,  balance,  and 
weights.  Follow  the  same  methods  and  record  the  facts 


GASES   AND    LIQUIDS  61 

observed  as  before.  Compare  the  records  with  those 
previously  obtained  for  the  cylinder  and  funnel. 

It  is  found  that  the  same  depth  of  water  is  required 
in  the  chimney  as  in  the  funnel  and  in  the  cylinder 
to  produce  the  same  pressure  on  the  brass  plate  at  the 
base.  This  is  true,  notwithstanding-  the  fact  that  the 
same  depth  of  water  in  the  case  of  the  chimney  requires 
a  considerably  smaller  volume  than  in  the  case  of  either 
the  funnel  or  the  cylinder.  Similar  results  are  obtained 
by  using-  vessels  very  irregular  in  form  but  having  an 
equal  area  at  the  base.  From  this  we  draw  the  conclu- 
sion, that  the  pressure  of  water  upon  any  horizontal  surface 
varies  with  the  depth  of  the  water  above  the  surface,  hut  is  in- 
dependent of  the  volume  of  water  used.  On  the  same  sur- 
face a  small  volume  of  water  produces  as  great  a  pressure 
as  an  enormously  greater  quantity  of  water,  provided 
that  the  depth  be  the  same.  To  produce  this  effect  in  a 
striking  manner,  the  smaller  quantity  of  water  must  be 
placed  in  a  vessel  having  the  same  diameter  at  the  base, 
but  a  diameter  at  the  top  much  smaller  than  in  the  vessel 
used  for  the  larger  quantity  of  water. 

49.  The  Upward  Pressure  of  a  Liquid  on  Any  Area  Is 
Equal  to  its  Downward  Pressure  on  the  Same  Area  at  the 
Same  Distance  beneath  the  Surface. — Take  three  sticks, 
each  of  them  one  inch  square,  but  respectively  4,  6,  and 
8  inches  in  length.  Bore  a  hole  lengthwise  in  each  stick 
to  within  about  a  quarter  of  an  inch  of  the  opposite  end. 
Put  in  shot  as  ballast.  Place  two  or  three  round  pieces 
of  pasteboard  over  the  shot  and  ram  the  shot  down  tight. 
Close  the  openings  with  cork.  Cut  off  the  cork  even  with 
the  surface  of  the  wood.  Let  the  total  weight  of  the 
sticks,  including  the  shot,  pasteboard,  and  cork,  be  2,  3, 
and  4  ounces  respectively.  Place  the  three  sticks  under 


ELEMENTARY   PHYSICS 


a  bell-jar  and  pump  out  the  air  in  the  jar.  A  great  por- 
tion of  the  air  enclosed  in  the  fibres  of  the  wood  is  re- 
moved at  the  same  time.  As  soon  as  possible  after 
pumping-  out  the  air,  boil  the  sticks  for  a  short  time  in 
hot,  melted  paraffine,  so  that  the  surface  of  the  wood 
shall  be  soaked  with  parafftne.  This  will  prevent  the 
sticks  from  becoming  water-logged. 

Place  the  three  sticks  in  a  tall  glass  jar  containing 
water  (Fig.  46).  Notice  the  depth  to  which  each  sinks 
into  the  water.  The  base  of  the  2-ounce  stick  is  about  3| 
inches  below,  the  base  of  the  3- 
ounce  stick  is  about  5j  inches  be- 
low, and  the  base  of  the  4-ounce 
stick  is  about  7  inches  below  the 
surface  of  the  water.  The  sticks 
press  downward  on  the  water 
upon  which  they  rest  with  a  force 
of  2,  3,  and  4  ounces  respectively. 
Nevertheless,  they  do  not  sink  to 
the  bottom  of  the  vessel.  The 
water  upon  which  the  sticks  rest 
is  in  each  case  pressing  upward 
with  the  same  force  with  which  the  sticks  are  pressing 
downward  ;  otherwise  the  sticks  would  either  rise  or  sink. 
The  upward  pressure  of  water  on  a  surface  one  inch 
square  is,  therefore,  2  ounces,  3  ounces,  and  4  ounces  at 
levels  approximately  3|,  5j,  and  7  inches  below  the  sur- 
face. 

The  same  result  will  be  obtained  no  matter  whether 
the  sticks  be  placed  in  a  shallow  or  in  a  deep  vessel. 
The  upward  pressure  of  water  on  any  area  immersed  in 
it  depends,  therefore,  not  upon  the  depth  of  the  water 
beneath  the  area  pressed  upon,  but  upon  the  depth  of 


FIG.  46. 


GASES   AND   LIQUIDS  63 

the  water  above  this  area ;  in  other  words,  upon  the  dis- 
tance of  this  area  beneath  the  surface  of  the  water. 

The  downward  pressure  of  water  on  any  area  is  equal 
to  the  weight  of  a  column  of  the  water  having-  vertical 
walls  and  having-  an  equal  area  as  a  base  (§  47).  This 
downward  pressure  may  be  determined  mathematically 
by  finding-  the  area  pressed  upon,  in  square  inches,  then 
multiplying-  this  by  the  distance  of  the  area  below  the 
surface  of  the  water  expressed  in  inches,  and  finally  mul- 
tiplying the  product  by  the  weight  of  a  cubic  inch  of 
water. 

A  cubic  inch  of  water  weighs  .577+  ounce  (§  57).  Cal- 
culate the  downward  pressures  of  water  on  areas  one  inch 
square,  at  depths  of  3J,  5j,  and  7  inches.  The  downward 
pressures  are  approximately  2,  3,  and  4  ounces  respec- 
tively. 

Compare  the  downward  pressures,  obtained  by  calcula- 
tion, with  the  upward  pressures  as  determined  in  the 
present  experiment.  It  will  be  found  that  on  the  same 
areas  placed  at  the  same  depths  beneath  the  surface  of 
the  liquid,  the  upward  and  downward  pressures  are 
equal.  This  fact  is  of  the  greatest  importance  in  ex- 
plaining the  apparent  loss  in  weight  of  bodies  immersed 
in  liquids  (§  51). 

50.  Loss  of  Weight  of  a  Body  Immersed  in  a  Liquid.— 
Attach  a  stone  by  means  of  a  long  string  to  a  spring  bal- 
ance, and  find  its  weight.  Hold  the  balance  over  a  jar  of 
water,  lower  the  stone  into  the  water,  and  determine  its 
weight  again  (Fig.  47).  Notice  that  the  stone  apparently 
loses  in  weight  when  immersed  in  water.  Place  your 
hand  beneath  the  body  of  a  person  swimming  in  water. 
Notice  how  easily  you  can  support  his  weight  though 
using  only  a  finger  or  two  (§  52). 


64 


ELEMENTARY   PHYSICS 


51.  Cause  of  This  Apparent  Loss  in  Weight. — Place  a 
rectangular  block  of  wood  or  of  any  other  substance  in  a 

vertical  position  beneath  the 
surface  of  the  water  in  a  glass 
jar  (Fig.  48).  The  only  direction 
in  which  water  can  press  upon 
the  top  of  the  block  is  down- 
ward ;  the  only  direction  in 
which  it  can  press  against  the 
sides  is  lateral;  and  the  only 
direction  in  which  it  can  press 
against  the  base  is  upward. 
The  various  pressures  against 
the  sides  balance  each  other. 
This  is  shown  by  the  fact  that 
the  block  is  not  pushed  toward 
any  side,  but  remains  wherever 
placed.  If  the  pressures  are 
unequal,  the  block  is  pushed  in 
a  direction  from  the  stronger 
force  toward  the  weaker  force. 
The  downward 
and  upward 

pressures  cannot  balance  one  another, 
since  the  upward  pressure  which  is  ex- 
erted against  the  bottom  of  the.block  at  a 
lower  depth  is  greater  than  the  downward 
pressure  which  is  exerted  on  the  top  of 
the  block  at  a  less  depth  beneath  the 
surface  of  the  water. 

If  the  upper  surface  of  the  block  have 
an  area  of  2  square  inches,  and  if  this  top  be  5  inches 
below  the  surface  of  the  water,  the  downward  pressure  of 


FIG.  47. 


FIG.  48. 


GASES   AND   LIQUIDS  65 

the  water  on  the  top  of  the  block  is  about  5.75  ounces  (§  47). 
If  the  base  be  of  equal  area,  and  if  it  be  8  inches  below  the 
surface  of  the  water,  the  upward  pressure  of  the  water  on 
the  base  will  be  about  9.25  ounces  (§  49).  In  this  case  the 
upward  pressure  will  be  3.5  ounces  greater  than  the  down- 
ward pressure.  If,  therefore,  the  block  weighs  less  than 
3.5  ounces,  it  will  rise  and  float;  if  it  weighs  exactly  3.5 
ounces,  it  will  remain  in  the  position  in  which  it  was 
placed ;  and  if  it  weighs  more  than  3.5  ounces,  it  will 
sink. 

Calculate  the  weight  of  the  volume  of  water  pushed 
aside  by  the  block.  Its  volume  is  2  X  3  =  6  cubic  inches. 
Its  weight  is  6  X  .577+  =  3.5  ounces  approximately. 
Therefore,  in  order  that  a  body  may  sink,  it  must  weigh 
more  than  an  equal  volume  of  water. 

Even  though  a  body  sinks  in  water,  and  thus  shows 
that  it  is  heavier  than  the  water,  it  nevertheless  appar- 
ently loses  in  weight.  This  is  necessarily  true  since 
the  upward  pressure  against  the  base  of  the  block  im- 
mersed is  always  greater  than  the  downward  pressure 
on  the  top.  An  examination  of  the  numbers  in  the  pre- 
ceding paragraph  shows  that  a  body  immersed  in  a 
liquid  apparently  loses  in  weight  an  amount  equal  to  the 
weight  of  a  quantity  of  the  liquid  whose  volume  is  equal 
to  the  volume  of  the  body  immersed. 

In  ordinary  language  we  say  that  a  body  immersed  in 
a  liquid  loses  in  weight  because  of  the  buoyancy  of 
the  liquid.  Buoyancy  is  not  a  separate  force.  It  is  sim- 
ply the  effect  produced  by  the  inequality  between  the 
downward  and  upward  pressures  exerted  by  a  liquid 
upon  any  immersed  body. 

52.  Buoyancy  of  Gases. — Loss  in  weight  occurs  also 
when  a  body  is  immersed  in  a  gas.  The  loss  is  usually 


66  ELEMENTARY   PHYSICS 

small,  because  there  is  but  a  small  difference  between  the 
downward  and  the  upward  pressures  of  gases  on  areas  at 
nearly  the  same  level.  The  loss  in  weight  can  easily 
be  detected  by  means  of  a  delicate  balance.  A  body 
weighed  in  air  weighs  less  than  the  same  body  weighed 
in  a  vacuum.  A  boy  weighing  125  pounds  in  a  vacuum 
will  weigh  about  2|  ounces  less  when  weighed  in  air. 
In  estimating  the  weight  of  the  boy,  the  loss  in  weight 
due  to  immersion  in  air  is  never  considered. 

The  loss  of  weight  in  water  is  much  more  likely  to  be 
appreciated,  especially  by  boys  who  are  in  the  habit  of 
swimming.  A  boy  will  lose  by  far  the  greater  part  of  his 
weight  if  he  is  weighed  in  water.  If  his  lungs  be  kept 
well  inflated  with  air,  his  weight  in  water  will  be  so  small 
that  a  slight  exertion  on  his  part  will  enable  him  to  float. 
Some  persons  can  float,  apparently  without  exertion, 
while  lying  in  the  water,  back  downward,  with  their  lungs 
well  inflated. 

53.  Pascal's  Law. — Select  a  bottle  with  a  neck  of  such 
an  even  bore,  that  a  cork  of  the  same  diameter  can  be 
moved  up  and  down  in  the  neck  like  a  piston  in  a  cylin- 
der. Fill  the  bottle  with  water.  Insert  the  cork.  Since 
the  cork  fits  tightly,  no  water  can  escape.  Suppose  that 
the  neck  of  the  bottle  is  of  such  a  diameter  that,  when 
the  bottle  is  filled,  the  surface  of  the  water  within  the 
neck  has  an  area  of  one  square  inch.  Then  the  cork 
inserted  in  the  neck  rests  upon  a  water  surface  of  one 
square  inch.  Suppose  the  cork  to  be  pressed  down  with 
a  force  of  one  pound  upon  the  square  inch  of  water  sur- 
face with  which  it  comes  in  contact  in  the  neck.  Then 
imagine  a  hole  with  an  area  of  one  square  inch  to  be  cut 
in  the  side  of  the  bottle.  It  is  evident  that,  disregarding 
the  pressure  due  to  the  weight  of  the  water,  a  pressure 


GASES  AND   LIQUIDS 


67 


of  one  pound  is  required  to  keep  the  water  from  running 
out  of  this  hole.  If  the  number  of  holes  be  increased,  a 
pressure  of  one  pound  will  be  re- 
quired on  each  hole  to  prevent  the 
escape  of  the  water  (Fig1.  49).  In 
other  words,  every  square  inch  of 
surface  within  the  bottle  will  sustain 
a  pressure  of  one  pound,  in  addition 
to  the  pressure  which  it  already  sus- 
tains due  to  the  weight  of  the  water 
alone. 

The  principle  here  involved  may  be 
stated  as  a  general  rule  known  as 
Pascal's  Law. 

Fluids  enclosed  in  a  vessel,  when  sub- 
jected to  pressure,  transmit  this  pressure 
undiminished  in  all  directions  so  that 
every  surface  in  the  interior  of  the  ves- 
sel, equal  in  area  to  the  surface  upon 
which  the  pressure  is  exerted,  receives  a  pressure  equal  to 
that  applied. 

In  case  a  wide  and  a  narrow  vessel  are  connected  by  a 
tube  at  the  bottom  and  almost  filled  with  water,  the  level 
of  the  water  in  the  two  vessels  is  the  same.  If  a  movable 
piston  be  placed  on  the  surface  of  the  water  in  each  ves- 
sel, and  a  weight  be  placed  on  the  smaller  piston,  this 
weight  will  balance  a  weight  placed  on  the  larger  piston 
as  many  times  greater  as  the  area  of  the  water  surface  of 
the  wider  vessel  exceeds  that  of  the  narrower  one. 

It  is  difficult  to  make  clear  the  reasons  for  the  facts 
here  presented.  It  may  be  best  for  the  present  to  omit 
any  explanation.  In  the  future,  when  the  student  knows 
more  about  molecules  and  the  laws  governing  their  ac- 


FiG.  49. 


ELEMENTARY  PHYSICS 


tion,  the  subject  can  be  studied  with  more  profit.  The 
practical  application  of  this  law  is,  however,  of  consider- 
able importance. 

54.  Hydrostatic  Pressure  TTsed  to  Lift  Weights.— Take  a 
short  brass  cylinder,  having  a  diameter  of  nearly  six  inches, 
with  an  opening  through  its 
base  into  which  is  inserted 
the  end  of  a  short  brass  tube, 
and  with  a  brass  plate  which 
works  up  and  down  in  the 


FIG.  50. 

cylinder  like  a  piston.     This  is  the  same  apparatus  as 
that  described  in  paragraph  12,  but  inverted. 

Carry  the  apparatus  into  the  hall  and  place  it  near  the 
staircase.  Thrust  the  plate  down  against  the  bottom 
of  the  cylinder.  Place  upon  it  a  block  of  wood  high 
enough  to  project  slightly  above  the  margin  of  the  cylin- 
der. On  top  of  the  block  put  a  board  and  ask  one  of 
the  pupils  to  stand  upon  it.  Slip  one  end  of  a  short  piece 


GASES   AND   LIQUIDS 


69 


of  rubber  tubing  over  the  brass  tube  at  the  base  of  the 
cylinder  and  into  the  other  end  insert  a  stop-cock.  Attach 
to  the  other  end  of  the  stop-cock  about  thirty  feet  of 
stout  rubber  tubing  filled  with  water,  and  into  the  farther 
end  of  the  tubing  insert  a  funnel. 

Close  the  stop-cock.  Pour  sufficient  water  into  the 
funnel  to  nearly  fill  it.  Carry  this  end  of  the  tube  to  the 
top  of  the  stairway.  Open  the  stop-cock 
(Fig.  51).  The  water  escapes  into  the 
lower  part  of  the  cylinder  and  pushes 
upward  against  the  base  of  the  brass 
plate.  The  pupil  is  lifted  up  very  slowly 
but  steadily.  If  the  funnel  be  kept  full 
of  water,  the  pupil  will  continue  to  be 
lifted  until  the  plate  reaches  the  top  of 
the  cylinder. 

The  pressure  of  the  water  on  the  area 
at  the  bottom  of  the  rubber  tubing  is 
quite  small.  This  is  because  the  area 
exposed  here  is  quite  small.  The  area 
of  the  brass  plate  is  several  hundred 
times  larger  than  the  area  at  the  base 
of  the  rubber  tube.  It  sustains,  there- 
fore, several  hundred  times  the  pressure 
sustained  by  the  area  at  the  base  of  this 
tube.  Although  the  downward  pressure  at  the  base  of 
the  narrow  tube  is  small,  the  upward  pressure  against  the 
broad  brass  plate  is  sufficient  to  lift  the  pupil. 

55.  Hydraulic  Press.— In  the  same  manner,  in  the  hy- 
draulic press,  the  pressure  exerted  upon  a  small  surface 
of  water  by  means  of  the  piston  of  a  sort  of  force-pump, 
is  transmitted  to  every  equal  area  on  the  base  of  a  very 
large  piston  (Fig.  52).  A  small  pressure  by  means  of  the 


FIG.  51. 


70 


ELEMENTARY  PHYSICS 


small  piston  causes  a  great  total  pressure  against  the 
much  larger  base  of  the  large  piston. 

The  principle  of  the  hydraulic  press  is  used  for  many 
purposes:   to  lift  bridges,  elevators,  and  even  ordinary 

JQL 


FIG.  52. 


barbers'  chairs.  The 
newest  forms  of  hoisting- 
jacks,  cider-presses,  and 
oil-presses  make  use  of 
the  same  principle. 

56.  Archimedes'  Prin- 
ciple. — An  overflow-can 
consists  of  a  brass  vessel 
about  3.5  inches  in  diameter  and  6  inches  high,  supplied 
near  the  rim  with  a  short,  nearly  horizontal  tube  which 
serves  as  a  spout.  Pour  water  into  the  can  until  it  runs 
out  through  the  spout.  After  the  water  has  ceased  flow- 
ing from  the  can,  place  beneath  the  spout  a  small  brass 
bucket  whose  weight  has  been  previously  determined  by 
means  of  a  spring  balance. 

Attach  a  rock  to  the  same  balance  by  means  of  a  thread. 
Weigh  the  rock  in  air,  then  lower  it  slowly  into  the  can 
and  weigh  it  again  in  the  water.  Determine  the  appar- 
ent loss  in  the  weight  of  the  rock  due  to  the  buoyant 


GASES  AND   LIQUIDS 


71 


Overflow  can 

FIG.  53. 


effect  of  the  water.  If  the  water  caught  in  the  bucket 
was  forced  out  of  the  overflow-can  only  in  consequence  of 
displacement  by  the  rock  (Fig-.  53) 
and  not  owing-  to  any  jerking  mo- 
tion during  the  weighing  of  the 
rock  in  the  water,  then  determine^ 
the  combined  weight  of  the  bucket 
and  of  the  water  which  has  run 
over  into  it  from  the  can. 

Next  find  the  weight  of  the 
water  displaced  by  the  stone,  by 
subtracting  the  weight  of  the 
bucket  alone  from  the  weight  of  the  bucket  and  the 
water.  Compare  the  loss  in  weight  of  the  rock  due  to 
buoyancy  with  the  weight  of  the  water  displaced  by  the 
rock.  They  are  the  same. 

From  this  we  learn,  that  a  body  immersed  in  a  liquid 
apparently  loses  in  weight  an  amount  equal  to  the  weight  of 
the  liquid  displaced. 

This  statement  of  fact  is  usually  known  as  Archimedes^ 
Principle. 

57.  Specific  Gravity. — The  specific  gravity  of  a  substance 
is  a  number  which  shows  how  many  times  heavier  that 
substance  is  than  an  equal  volume  of  another  substance 
taken  as  a  standard.  The  standard  for  solids  and  liquids 
is  distilled  water  at  a  temperature  of  4  degrees  Centi- 
grade, or  39.2  degrees  Fahrenheit.  A  reading  of  4  degrees 
on  the  Centigrade  thermometer  indicates  the  same  temper- 
ature as  that  indicated  by  the  reading  of  39.2  degrees  on 
the  Fahrenheit  thermometer.  At  this  temperature  water 
has  its  maximum  density  ;  in  other  words,  at  this  temper- 
ature a  given  weight  of  water  occupies  less  space  and 
therefore  is  more  dense  than  at  any  other  temperature. 


72  ELEMENTAEY   PHYSICS 

In  determining"  the  specific  gravity  of  gases,  hydrogen 
subjected  to  a  barometric  pressure  of  30  inches  (§  14)  is 
usually  taken  as  the  standard.  Sometimes  air  is  taken 
as  the  standard. 

Iron  has  a  specific  gravity  of  7.8.  This  means  that  any 
piece  of  pure  iron  is  7.8  times  as  heavy  as  an  equal  bulk 
of  water.  If,  for  instance,  it  is  desired  to  find  the  weight 
of  one  cubic  foot  of  iron,  it  is  sufficient  to  multiply  the 
weight  of  one  cubic  foot  of  water  by  7.8.  The  weight  of 
one  cubic  foot  of  water  has  been  found  by  careful  weigh- 
ing to  be  62.42  pounds  (§  49).  A  cubic  foot  of  iron, 
therefore,  weighs  487.88  pounds. 

58.  Specific  Gravity  of  Solids  Heavier  than  Water — How 
is  it  possible  to  determine  the  fact  that  the  specific  gravity 
of  iron  is  7.8  ?  Take  any  piece  of  pure  iron.  Weigh  it 
in  air.  Determine  the  weight  of  an  equal  volume  of 
water.  Divide  the  weight  of  the  iron  in  air  by  the  weight 
of  the  equal  volume  of  water.  The  quotient  is  the  specific 
gravity  of  iron.  To  determine  the  weight  of  an  equal 
volume  of  water,  weigh  the  water  which  escapes  into  the 
bucket  when  the  piece  of  iron  is  lowered  into  an  overflow- 
can  full  of  water. 

It  is  possible  to  secure  the  weight  of  an  equal  volume 
of  water  without  using  the  overflow-can  and  the  bucket. 
From  Archimedes'  Principle  we  know  that  the  apparent 
loss  in  weight  of  any  rock,  immersed  in  water,  is  equal 
to  the  weight  of  a  mass  of  water  equal  in  volume  to  the 
volume  of  the  rock.  It  is,  therefore,  sufficient  to  deter- 
mine the  apparent  loss  in  weight  of  the  rock  in  the  water, 
in  order  to  know  the  weight  of  an  equal  volume  of  water. 
For  instance,  if  the  rock  weighs  5  ounces  in  air  and  ap- 
parently 3  ounces  in  water,  the  so-called  loss  in  weight 
is  2  ounces.  Hence  the  weight  of  a  mass  of  water 


GASES   AND   LIQUIDS  73 

equal  in  volume  to  the  volume  of  the  rock  is  also  2 
ounces. 

If  the  rock  weighs  5  ounces  in  air  and  an  equal  bulk  of 
water  weighs  2  ounces,  the  rock  must  weigh  2.5  times  as 
much  as  an  equal  bulk  of  water. 

The  greater  weight  of  rock  as  compared  with  the 
weight  of  water  is  believed  to  be  due  to  the  fact  that 
there  is  actually  more  material  in  a  cubic  inch  of  rock 
than  in  a  cubic  inch  of  water.  Eock  is  believed  to  be 
more  compact,  or  more  dense.  In  accordance  with  this 
idea,  the  particular  kind  of  rock  here  used  is  said  to  be 
2.5  times  as  dense  as  water.  The  number  which  specifies 
how  the  density  of  any  substance  compares  with  the  den- 
sity of  water  is  called  the  Specific  Density  of  that  sub- 
stance. Therefore,  the  specific  density  of  the  stone  in 
the  preceding  example  is  2.5.  In  other  words,  its  den- 
sity is  2.5  as  great  as  the  density  of  water. 

Since  the  weight  of  a  body  is  proportional  to  the 
amount  of  material  in  it,  the  specific  gravity  of  a  sub- 
stance is  the  same  as  its  specific  density.  For  this  reason 
these  two  terms  are  often  used  interchangeably. 

The  method  used  in  determining  the  specific  gravity  of 
any  substance  depends  upon  its  weight,  solubility,  chem- 
ical action  when  exposed  to  air  or  immersed  in  water,  and 
other  properties.  A  few  additional  methods  are  described 
in  the  following  paragraphs.  The  underlying  principle 
in  all  of  these  methods  is  the  comparison  of  the  weights 
of  equal  bulks  of  these  substances  and  of  water. 

59.  Specific  Gravity  of  Solids  Lighter  than  Water.— 
Attach  a  small  block  of  wood  to  a  balance  and  weigh  it. 
To  the  block  fasten  a  stone  heavy  enough  to  drag  the 
block  beneath  the  surface  of  water.  Hold  the  spring 
balance  over  a  jar  of  water  and  lower  it  sufficiently  to 


74 


ELEMENTARY  PHYSICS 


permit  the  entire  stone  to  dip  beneath  the  surface  of  the 
water,  while  all  of  the  block  remains  in  the  air  (Fig.  54,  A). 
Determine  the  combined  weight  of  block  and  stone  in 
this  position.  Lower  the  balance  still  farther,  until  both 

block  and  stone  are  under  the 
surface  of  the  water  (Fig-.  54,  B). 
Ascertain  the  weight  of  both 
in  this  position  also.  The  loss 


I 


FIG.  51. 


FIG.  54. 


in  weight  of  the  combination  is  due  to  the  buoyant  effect 
of  the  water  upon  the  block  of  wood.  This  loss  in  weight, 
according  to  Archimedes'  Principle,  is  equal  to  the  weight 
of  a  quantity  of  water  whose  volume  is  equal  to  the  vol- 
ume of  the  block. 


GASES   AND   LIQUIDS  75 

If  the  combination,  while  the  block  is  in  air  and  the 
stone  is  in  water,  weighs  28  ounces,  and  the  combination 
when  the  block  is  also  under  water,  weighs  8  ounces, 
then  the  weight  of  a  volume  of  water  equal  to  the  volume 
of  the  block  is  20  ounces.  If  the  weight  of  the  block  alone 
is  14  ounces,  the  wood  is  .7  times  as  heavy,  bulk  for  bulk,  as 
water,  or  its  density  is  only  .7  times  as  great  as  that  of 
water.  In  other  words,  the  specific  gravity  or  the  specific 
density  of  the  kind  of  wood  investigated  is  .7. 

60.  Specific  Gravity  of  Liquids.— FIRST  METHOD.  Find 
the  weight  of  an  empty  bottle.  Weigh  the  bottle  when 
filled  with  water,  and  afterwards  weigh  it  again  when 
filled  with  the  liquid  whose  specific  gravity  is  to  be 
determined.  In  this  manner  the  weights  of  equal  vol- 
umes of  water  and  of  the  liquid  are  obtained.  To  find 
how  many  times  heavier  the  liquid  is  than  an  equal  vol- 
ume of  water,  divide  the  weight  of  the  liquid  by  the 
weight  of  the  water.  The  quotient  obtained  is  the 
specific  gravity  of  the  liquid. 

SECOND  METHOD.  Weigh  a  solid  in  air,  then  in  water, 
and  then  in  the  liquid.  The  loss  in  weight  of  the  solid 
when  weighed  in  water  and  the  loss  when  weighed  in  the 
liquid,  are  the  respective  weights  of  equal  bulks  of  the 
water  and  of  the  liquid  which  were  displaced  by  the 
solid.  Hence,  to  find  the  specific  gravity  of  the  liquid, 
divide  the  loss  in  weight  of  the  solid  when  weighed  in 
the  liquid  by  its  loss  in  weight  when  weighed  in  water. 


CHAPTEE  H 
MOLECULAR  PHENOMENA 

61.  The  Contraction  of  Liquids  Due  to  Loss  of  Heat  Indi- 
cates That  Liquids  Are  Made  Up  of  Particles  Called  Molecules. 
— Scientific  study  has  often  led  to  conclusions  widely  at 
variance  with  opinions  ordinarily  looked  upon  as  per- 
fectly simple  and  self-evident.  It  is  usually  taken  for 
granted,  for  example,  that  water  occupies  all  of  the  space 
which  it  seems  to  occupy.  Even  a  drop  of  water,  if  placed 
under  a  microscope,  appears  perfectly  compact,  and  re- 
veals no  empty  spaces  anywhere  within  the  liquid. 
Nevertheless,  certain  experiments  suggest  that  water  is 
made  up  of  particles  and  that  empty  spaces  actually  exist 
between  the  particles.  Among  these  experiments,  the 
following  are  simple  and  easily  performed. 

Fill  a  small  Florence  flask  with  water 
having  a  temperature  of  about  72  degrees 
Fahrenheit.  Insert  one  end  of  a  12-inch 
piece  of  fine  glass  tubing  through  the  hole 
in  a  rubber  stopper.  Force  the  stopper 
into  the  mouth  of  the  bottle,  until  the  water 
displaced  by  the  stopper  rises  nearly  to  the 
top  of  the  glass  tube  (Fig.  55).  The  water 
>e  rendered  more  readilv  visible  from 


FIG  55 

a  distance,  if,  before  it  is  placed  in  the  flask, 

it  is  colored  deeply  by  means  of  some  aniline  dye.     Cool 
the  water  in  the  flask  by  placing  it  in  ice-water.    The 

76 


MOLECULAR   PHENOMENA  77 

water  in  the  glass  tube  falls.  That  part  of  the  water, 
which  before  cooling  filled  the  flask,  must  now  occupy 
less  space,  in  order  to  permit  a  part  of  the  water  which 
was  forced  up  into  the  tube  to  return  to  the  flask. 

The  total  amount  of  water  present  in  the  flask  and  tube 
is  the  same  both  before  and  after  cooling.  This  is  shown 
by  the  fact  that  in  a  vacuum  (§  52)  the  water  weighs  pre- 
cisely as  much  at  the  lower  as  at  the  higher  temperature. 
How,  then,  can  water  occupy  less  space  at  one  tempera- 
ture than  at  another  ?  It  has  been  suggested  that  water 
consists  of  separate  particles,  which  are  slightly  distant 
from  one  another,  and  that  cooling  is  a  process  which 
brings  the  particles  of  water  closer  together.  These 
separate  particles  have  received  the  name  of  molecules  of 
water. 

62.  Increase  of  Heat  Causes  the  Molecules  of  Liquids  to 
Move  Farther  Apart. — If  cooling  is  a  process  which  brings 
the  molecules  of  water  closer  together,  then  the  converse 
also  must  be  true,  that  heating  is  a  process  which  causes 
the  distance  between  the  molecules  of  water  to  increase. 

Prepare  the  apparatus  as  in  the  case  of  the  experiment 
described  in  the  preceding  paragraph.  Pour  enough 
water  into  the  flask  to  fill  the  flask  when  closed  and  to 
rise  in  the  tube  to  only  a  short  distance  above  the  level 
of  the  stopper.  Place  the  flask  on  a  piece  of  wire  gauze 
which  rests  on  an  iron  ring  stand.  Heat  it  gently  by 
means  of  a  Bunsen  burner  or  alcohol  lamp.  The  heat 
affects  the  glass  before  it  reaches  the  water.  The  mole- 
cules of  glass  separate,  the  flask  becomes  slightly  larger, 
and  the  water  in  the  tube,  which  is  still  cool,  falls. 
However,  for  the  same  increase  in  temperature,  the  in- 
crease in  volume  of  the  water  is  much  greater  than  the 
increase  in  volume  of  the  glass.  Therefore,  as  soon  as 


78  ELEMENTAKY   PHYSICS 

the  water  in  the  flask  is  also  affected  by  the  heat,  the  dis- 
tance between  the  molecules  of  water  also  increases,  and 
the  water  soon  rises  rapidly  in  the  tube  until  it  runs  out 
at  the  top  (§  120). 

63.  Solids  are  Composed  of  Molecules. — The  ball  and  ring 
apparatus  consists  of  a  copper  ball  attached  by  a  chain  to 

a  wooden  handle,  and  of  a  cop- 
per ring-  fastened  at  the  end  of 
a  rod  which  is  also  supplied 

__         __aaf-«**r-^  w^  a  lian(^e-    When  both  ball 
~"*sssss^  and  ring  are  at  the  same  tem- 
perature, the  ball  will  barely  pass  through  the 
ring  (Fig.  56). 

If  the  ring  is   cooled  by  means  of  ice  or  ice- 
water,  the  ball  can  no  longer  pass  through  the 
FIG.  56.   ring.     Cooling  has  caused  the  molecules  of  cop- 
per to  move  closer  together.     The  copper  in  the 
ring  now  occupies  less  space,  and,  in  consequence,  the 
diameter  of  the  ring  is  shortened.     The  decrease  in  the 
diameter  of  the  opening  through  the  ring  may  be  meas- 
ured by  means  of  a  vernier  sliding  caliper. 

Permit  the  ring  to  return  to  its  original  temperature. 
Then  heat  the  ball.  It  increases  in  size.  The  ball  is 
now  too  large  to  pass  through  the  ring.  If,  however, 
the  ring  also  is  heated  sufficiently,  it  becomes  so  en- 
larged that  the  heated  ball  can  once  more  pass  through  it. 
It  is  necessary  to  come  to  the  conclusion  that  copper  as 
well  as  water  is  composed  of  particles  or  molecules  which 
are  not  in  direct  contact  with  one  another.  Since  the 
separate  particles  have  never  been  seen,  even  by  means 
of  the  strongest  microscope,  the  molecules  of  both  sub- 
stances must  be  exceedingly  small,  and  the  distance  be- 
tween them  must  be  very  minute. 


MOLECULAR  PHENOMENA  79 

64.  The  Distance  between  the  Molecules  of  Some  Liquids 
Is  Sufficient  to  Permit  the  Interposition  of  Molecules  of  Other 
Liquids. — If  water  consists  of  molecules  and  these  mole- 
cules are  separated  by  very  minute,  but  none  the  less  real 
spaces,  the  question  arises  whether  it  is  possible  to  insert 
any  substances  into  the  spaces  between  the  molecules  of 
water.  The  following  experiment  suggests  that  this  may 
be  accomplished  successfully. 

Pour  water  into  a  long  tube  which  is  closed  at  one  end 
( for  example,  a  combustion-tube  about  half  an  inch  in 
diameter),  until  it  rises  to  a  level  of  7  inches  above  the 
base  of  the  tube.  Pour  7  inches  of  alcohol  on  top 
of  the  water,  so  gently  that  the  liquids  do  not 
commingle  (Fig.  57,  A).  Mark  the  upper  limit  of 
the  alcohol  by  a  rubber  band  slipped  over  the 
top  of  the  tube.  The  water  and  alcohol  seem  to  ^|||  ||| 
occupy  all  the  space  in  the  tube  for  a  length  of 
exactly  14  inches.  However,  when  the  opening 
at  the  top  of  the  tube  is  closed  by  means  of  the 
thumb,  and  the  tube  is  repeatedly  inverted  and 

turned  back  to  its  original  position,  the  volume 

FIG.  57. 
of  the  mixture  is  decreased  to  such  an  extent 

that  the  length  of  the  tube  occupied  by  the  mixture  is 
shortened  by  about  three-tenths  of  an  inch  (Fig.  57,  B). 
For  some  reason  the  two  liquids,  when  mixed,  occupy 
less  space  than  when  kept  separate. 

This  shortening  of  the  liquid  column  can  be  explained 
only  by  the  supposition  that  the  molecules  of  one  liquid 
may  slip  into  the  empty  spaces  which  exist  between  the 
molecules  of  the  other  liquid.  Possibly  the  spaces  be- 
tween the  molecules  in  both  liquids  are  large  enough  to 
permit  in  each  case  the  entrance  of  some  of  the  molecules 
of  the  other  liquid. 


80  ELEMENTARY   PHYSICS 

65.  Small  Size  of  Molecules  Indicated  by  Gold  Leaf  and  by 
Soap-bubble  Film. — The  smallest  particles  which  can  be 
detected  by  means  of  the  best  microscopes  have  a  diameter 
of  about  jooloo  °^  an  incn-  Gold  has  been  hammered  into 
leaves  so  thin  that  the  thickness  of  a  single  leaf  does  not 
exceed  gooijoo  Par*  °^  an  incn-  ^  ^  were  possible  to  remove 
from  this  leaf  a  piece  whose  width  and  length  did  not  ex- 
ceed its  thickness,  the  piece  would  be  invisible  even  if 
search  were  made  with  the  best  microscope. 

A  few  moments  after  a  soap-bubble  has  been  blown, 
rings  of  color  make  their  appearance  at  the  top  of  the 
bubble  and  travel  down  its  sides.  Just  before  bursting, 
a  black  spot  becomes  visible  at  the  top  of  the  bubble, 
this  spreads,  and  a  moment  later  the  bubble  breaks.  It 
has  been  determined  by  investigators  in  physics  that  the 
thickness  of  the  colored  part  of  the  bubble  immediately 
below  the  margin  of  the  black  area,  is  about  ^TQQ  part  of 
an  inch.  The  black  part  of  the  soap-bubble  (just  before 
bursting)  is  much  thinner — according  to  some  investi- 
gators, about  g^o^ooo  of  an  inch.  This  thickness  is  about 
100  times  as  small  as  the  diameter  of  the  smallest  particle 
visible  by  means  of  the  best  microscope. 

Although  the  black  part  of  the  film  breaks,  the  cohe- 
sion between  the  molecules  composing  it  is  sufficiently 
strong  to  enable  the  film  to  remain  in  existence  for  an 
appreciable,  although  very  brief,  amount  of  time.  It  is 
difficult  to  conceive  of  even  this  degree  of  cohesion,  unless 
the  film  consists  of  several  layers  of  molecules. 

If  it  be  assumed  that  the  number  of  layers  of  molecules 
forming  the  film  is  at  least  equal  to  five,  then  the  di- 
ameter of  the  molecules  forming  the  film  does  not  exceed 
soWooo  °f  an  incn-  Molecules,  however,  are  supposed  not 
to  be  in  direct  contact  with  one  another,  at  least  not 


MOLECULAR  PHENOMENA  81 

for  any  appreciable  length  of  time.  Therefore  the  actual 
diameter  of  molecules  must  be  much  smaller  than  ^  ^  000 
of  an  inch. 

From  recent  mathematical  investigations  it  appears 
that  a  single  molecule  of  water  taken  from  a  soap  film 
may  be  as  small  as  250^00^000  °^  an  inch  in  diameter. 

66.  Relative  Size  of  Molecules.— Molecules  of  the  same 
substance  and  under  the  same  conditions  are  believed  to 
be  similar  in  every  respect.     The  molecules  of  one  kind 
of  substance,  however,  are  believed  to  differ  from  those  of 
another  kind.     Among  other  things,  they  differ  in  size 
and  in  weight.     For  instance,  the  molecules  of  mercury 
are  probably  larger  and  heavier  than  those  of  water. 

The  actual  size  of  molecules  is  unknown.  According 
to  some  calculations,  difficult  to  explain  here,  the  average 
diameter  of  molecules  is  about  62  5UU  000  of  an  inch.  It  has 
been  calculated  that  if  a  globe  of  water,  the  size  of  a  foot- 
ball six  inches  in  diameter,  were  magnified  to  the  size  of 
the  earth,  the  molecules  of  water  would  occupy  spaces 
greater  than  those  filled  by  small  shot  and  less  than 
those  filled  by  footballs.  These  ideas  may  not  be  very 
definite,  but  they  are  the  most  accurate  which  the  present 
state  of  science  affords  (§  78,  81). 

67.  Use  of  Litmus  in  Determining  the  Presence  of  Acids 
or  Alkalis. — The  value  of  the  evidence  furnished  by  the 
experiment  described  in   the   next    paragraph    will   be 
better  appreciated  if  the  use  of  litmus  in  chemical  ex- 
periments be  well  understood. 

The  color  of  litmus  is  blue  tinged  with  purple.  It  may 
be  purchased  in  the  form  of  small  cubes  which  may  be 
dissolved  in  water,  giving  to  the  water  a  characteristic 
blue  color.  A  single  drop  of  any  acid  will  change  the 
color  of  a  considerable  quantity  of  the  litmus  solution 


ELEMENTARY   PHYSICS 


from  blue  to  red.  The  addition  of  several  drops  of  any 
alkali  will  change  the  color  back  to  blue  once  more. 
Any  change  to  a  red  color  in  a  litmus  solution  originally 
blue  indicates,  therefore,  the  introduction  of  an  acid. 
Any  change  from  red  to  blue  indicates  the  introduction 
of  an  alkali. 

The  more  common  acids  are  sulphuric  acid,  hydro- 
chloric acid,  and  nitric  acid.  Among  the  common  alkalis 
are  ammonia  water  and  caustic  potash. 

68.  Diffusion  of  Liquids. — Fasten  a  long  test-tube,  one 
inch  in  diameter,  in  a  vertical  position.  Nearly  fill  it 
with  water  distinctly  colored  with  blue 
litmus.  Insert  a  thistle-tube  so  that  its 
lower  end  rests  upon  the  bottom  of  the 
test-tube.  Pour  hydrochloric  acid  into 
the  bowl  of  the  thistle-tube  so  slowly 
that,  when  the  acid  reaches  the  bottom 
of  the  thistle-tube,  its  entrance  will  not 
produce  a  perceptible  current  in  the 
colored  water  within  the  test-tube.  As 
soon  as  the  acid  reaches  the  bottom  of 
the  test-tube,  its  presence  is  at  once 
detected  by  the  change,  from  blue  to 
red,  in  the  color  of  the  litmus  dissolved 
in  the  water  (Fig.  58). 

Continue  to  add  acid  to  the  thistle- 
tube  until  the  red  color  extends  a  little 
over  half  an  inch  above  the  bottom  of  the  test-tube. 
Allow  the  apparatus  to  stand  for  several  days,  without 
disturbing  either  the  liquids  or  the  thistle-tube.  The  red 
coloring  gradually  extends  to  higher  and  higher  levels. 
This  indicates  that  the  molecules  of  the  acid  are  slowly 
travelling  upward  into  the  litmus  solution.  If  for  sev- 


FIG.  58. 


MOLECULAK  PHENOMENA  83 

eral  days  a  record  be  kept  of  the  height  to  which  the  red 
coloring  has  crept,  it  will  be  found  possible  to  determine 
the  rate  at  which  the  molecules  of  acid  travel  up  the 
tubes. 

Different  substances  travel  at  various  rates  of  speed. 
Hydrochloric  acid  travels  twice  as  fast  as  salt,  salt  travels 
three  times  as  fast  as  sugar,  and  sugar  travels  seven  times 
as  fast  as  the  albumen  of  an  egg.  The  name  albumen  is 
given  by  physiologists  to  the  colorless  liquid  which  sur- 
rounds the  yolk  of  an  egg,  and  which  after  cooking  be- 
comes hard  and  white. 

Two  liquids  in  contact  with  one  another  may  become 
intermingled  owing  to  the  wandering  of  their  molecules 
(§71).  This  slow  process  of  intermingling  is  known  as 
diffusion. 

69.  Membranes  for  Illustrating  Osmose   of  Liquids.— In 
many  cases  molecules  pass  readily  through  membranes 
in  which  even  the  best  microscope  can  detect  no  open- 
ings.    For  experimental  purposes,  the  best  membrane  is 
the  very  thin  membrane  which  lines  the  large  masses  of 
suet  (fat)  found  in  the  interior  of  the  bodies  of  cattle. 
These   membranes   may  be    easily    obtained  from   any 
butcher.     By  exercise  of   considerable   care,  this  mem- 
brane can  be  removed  without  rents  or  punctures. 

70.  Osmose  of  Liquids. — If  two  different  liquids  or  solu- 
tions are  placed  on  opposite  sides  of  a  membrane,  the 
molecules  of  both  liquids  or  solutions  may  pass  through 
the  membrane  at  the  same  time,  but  in  opposite  direc- 
tions.    This  may  be  shown  by  the  following  experiment. 

Cover  the  open  end  of  the  bowl  of  a  thistle-tube  with 
a  membrane  and  tie  it  securely  by  means  of  a  string 
wrapped  around  the  neck  of  the  bowl.  Invert  the  thistle- 
tube  and  insert  it  vertically  in  a  jar  nearly  filled  with 


84 


ELEMENTARY   PHYSICS 


water.    Fasten  the   thistle-tube  securely  in    this    posi- 
tion. 

Dissolve  in  water  all  the  (blue)  copper  sulphate  which 
the  water  will  hold.  If  the  solution  is  not  clear  add  a 
few  drops  of  sulphuric  acid.  Through  a  funnel  pour  the 
solution  into  the  upper  end  of  the  thistle-tube  (the  nar- 
row end,  opposite  the  bowl,  in  the  present  position  of 
the  tube),  until  the  level  of  the  copper  sulphate  solution 
in  the  tube  is  the  same  as  the  level  of  the  water  in  the 
jar.  The  level  of  the  copper  sulphate  solution  may  easily 
be  adjusted  to  that  of  the  water  by  first  bringing-' the  two 
liquids  approximately  to  the  same  level,  and  then  raising 
or  lowering  the  thistle-tube  until  the  surface  of  both 
liquids  is  found  at  exactly  the  same  level.  Then  allow 
the  apparatus  to  remain  undis- 
turbed for  several  hours. 

In  a  short  time  the  blue  copper- 
sulphate  solution  in  the  tube  rises 
above  the  level  of  the  water  in  the 
jar,  and  the  height  reached  by  the 
solution  increases  for  several  hours 
(Fig.  59).  The  rise  of  the  copper- 
sulphate  solution  in  the  thistle- tube 
can  be  due  only  to  the  passage  of 
water  from  the  jar,  through  the 
membrane,  up  into  the  bowl  of  the 
thistle-tube. 

At  the  same  time  that  part  of  the 
water  immediately  below  the  thistle- 
tube  becomes  tinged  with  light 
blue.  This  indicates  that  part  of 

the  copper-sulphate  solution  is  passing  from  the  bowl, 
through  the  membrane,  into  the  water  in  the  jar. 


MOLECULAR  PHENOMENA  85 

The  interchange  of  the  molecules  of  different  liquids 
through  the  same  membrane  in  opposite  directions  at 
the  same  time  is  known  as  the  osmose  of  liquids. 

71.  Varying  Rates  of  Transference  of  Molecules  through 
Membranes  during  Osmose  of  Liquids. — If  the  molecules 
of  copper  sulphate  passed  downward  through  the  mem- 
brane as  rapidly  as  the  molecules  of  water  move  in  the 
opposite  direction,  the  level  of  the  solution  in  the  thistle- 
tube  would  not  change.     In  the  experiment  described  in 
the  preceding  paragraph,  however,  the  level  of  the  copper 
sulphate  rises  and  that  of  the  water  falls.     Owing  to  the 
much  greater  width  of  the  jar,  the  fall  of  the  water  level 
is  practically  imperceptible.     This  change  in  level  of  the 
two  liquids  is  evidently  due  to  the  fact  that  the  mole- 
cules of  water  pass  through  the  membrane  more  rapidly 
than  do  the  molecules  of  copper  sulphate. 

The  comparative  rates  of  speed  shown  during  the  os- 
mose of  various  liquids  and  solutions  has  been  studied. 
Thus,  for  instance,  water  passes  through  a  membrane 
four  to  six  times  as  rapidly  as  salt.  Since  a  salt  solu- 
tion is  colorless,  its  motion  down  into  the  water  cannot 
be  made  visible,  and  for  this  reason  copper  sulphate 
was  used  in  the  preceding  experiment  (see  also  close  of 
§68). 

72.  Osmose   of   Gases. — Molecules  of  gases  are  much 
more  free  to  move  than  molecules  of  liquids.     If  there  be 
any  cohesion  between  the  molecules  of  a  gas,  it  is  so 
slight  that  it  is  impossible  to  detect  this  cohesion  experi- 
mentally. 

The  molecules  of  different  gases  move  with  different 
rates  of  speed.  This  may  be  shown  by  an  experiment 
very  similar  to  the  one  preceding. 

Cement  to  the  upper  edge  of  a  funnel  the  open  end  of 


8G 


ELEMENTAEY  PHYSICS 


a  long-,  cylindrical,  unglazed  earthen-ware  jar,  similar  to 
the  jars  used  in  two-fluid  electric  cells.  Thrust  the  tube 
of  the  funnel  vertically  through  the  opening  in  the  rubber 
stopper  which  closes  one  of  the  necks  of  a  two-necked 
bottle.  Nearly  fill  the  bottle  with  water.  Through  the 
rubber  stopper  in  the  second  neck  thrust  a  glass  tube 

until  the  lower  end  of  the 
tube  dips  beneath  the  surface 
of  the  water. 

The  experiment  will  prove 
more  interesting  if  before  in- 
serting the  glass  tube  in  the 
stopper,  the  upper  end  of 
the  tube  be  drawn  out  until 
the  opening  becomes  very 
small,  as  directed  in  §  36. 

Fill  a  bell -jar  with  hydro- 
gen (a  gas  already  mentioned 
in  §§  2, 3  and  4 ;  see  also  §§  137, 
138).  Lift  the  bell-jar  and, 
without  disturbing  its  ver- 
tical position,  place  it  above 
the  apparatus  and  lower  it 

^^  over    the    earthen -ware  jar 

(Fig.  60).   The  hydrogen  now 

enters  the  earthen- ware  jar  so  much  more  rapidly  than  the 
air  in  the  jar  can  move  out  in  the  opposite  direction,  that 
the  total  quantity  of  gas  within  the  earthen- ware  jar  is 
increased.  In  consequence,  the  pressure  of  the  mixture 
of  gases  in  the  jar  also  increases.  This  mixture  of  gases 
presses  downward  upon  the  water  in  the  bottle  with  such 
force  that  some  of  the  water  passes  up  the  glass  tube  and 
but  through  the  small  opening  in  the  form  of  a  tiny  spray. 


FIG.  GO. 


MOLECULAR  PHENOMENA  87 

73.  The  Rate  of  Motion  of  Molecules  of  Gases.  —  Mole- 
cules of  hydrogen  move  about  four  times  as  fast  as  the 
average  speed  of  the  molecules  forming  the  mixture  of 
gases  called  air.  It  has  been  calculated  that  hydrogen 
travels  at  the  rate  of  about  9  miles  per  second.  This 
does  not  mean  that  any  one  particle  of  hydrogen  actually 
travels  that  distance  in  a  straight  line  in  the  period  of 
one  second.  In  hydrogen  which  is  subjected  to  the  same 
pressure  as  ordinary  air,  the  molecules  are  probably  so 
close  together  that,  before  any  molecule  has  gone 


of  an  inch,  it  will  strike  some  other  molecule  of  hydrogen 
which  is  moving  in  some  other  direction.  While  the 
speed  with  which  a  molecule  of  hydrogen  travels  is  very 
great,  the  distance  which  it  gets  from  its  original  posi- 
tion in  one  second  is,  owing  to  these  collisions,  compara- 
tively small.  Still  the  total  change  in  position,  after  a 
number  of  seconds,  is  sufficiently  great  to  give  rise  to 
such  phenomena  as  those  just  described  in  the  preceding 
paragraph. 

74.  Evaporation.—  The  fact  that  the  molecules  of  a  liq- 
uid are  continually  changing  their  position  is  shown  by 
the  phenomena  known   as  diffusion.      Moreover,   many 
of  the  molecules  at  the   surface  of  a  liquid  leave  the 
remainder  of  the  liquid  and  move  out  into  the  air.     If 
these  molecules  do  not  return  to  the  liquid  from  which 
they  came,  the  quantity  of  liquid  contained  in  the  ves- 
sel is  permanently  diminished.      That  part  of  the  liquid 
which  disappears,  is  said  to  evaporate. 

75.  The   Evaporation    of  Solids,    or   Sublimation.  —  Liq- 
uids are  not  the  only  substances  which  evaporate.     If 
a  piece  of  camphor  be  exposed  to  the  air  for  any  length 
of  time,   it  will   evaporate  completely.     The  molecules 
leave  the  piece  of  camphor  and  wander  over  the  room: 


88  ELEMENTARY  PHYSICS 

They  take  the  form  of  a  vapor,  or  gas,  which  is  very  op- 
pressive to  certain  insects,  and  on  this  account  camphor 
is  often  placed  where  the  vapor  formed  by  its  evaporation 
will  prove  offensive  to  the  moths  which  attack  woollen 
clothing. 

Even  ice  evaporates.  If  a  piece  of  ice  be  exposed  to 
dry  air  in  a  place  where  the  temperature  is  much  below 
the  freezing  point,  it  will  gradually  disappear.  It  passes 
directly  into  the  state  of  a  vapor  without  first  taking  the 
liquid  state.  In  this  manner  a  thin  coating  of  ice  on  a 
stone  often  disappears  during  wintry  weather,  especially 
when  exposed  to  a  strong  wind.  When  clothing  just 
washed  is  hung  out  to  dry  on  days  when  the  temperature 
is  below  the  freezing  point,  the  water  in  it  freezes,  and 
the  clothing  becomes  stiff.  During  the  day,  however,  a 
large  part  of  the  ice  evaporates,  especially  in  case  a 
strong  wind  is  blowing. 

The  resins  and  oils  found  in  various  kinds  of  wood, 
such  as  pine  and  cedar,  also  evaporate  slowly.  This 
gives  rise  to  the  smell  which  is  characteristic  of  the 
various  kinds  of  wood. 

When  a  solid,  that  is  heated,  passes  directly  into  the 
state  of  a  vapor  without  first  becoming  liquid,  the  proc- 
ess is  called  sublimation.  A  cool  body  placed  in  the  path 
of  a  vapor  may  cause  it  to  return  again  to  the  solid  state, 
in  this  case  also  without  first  assuming  the  liquid  state. 

76.  The  Cause  of  the  Odor  of  the  Rose.— Investigators 
in  physiology  have  come  to  the  conclusion  that  we  can 
smell  only  that  with  which  we  actually  come  in  contact. 
We  say  that  we  smell  a  rose ;  but  in  reality  we  smell  only 
those  particles  which  leave  the  rose  and  strike  against 
our  nostrils. 
•  The  showy  part  of  the  flower  of  the  rose  consists  of  the 


MOLECULAR  PHENOMENA  89 

beautifully  colored  leaves,  called  petals.  Upon  these 
petals  are  minute  drops  of  oil.  The  oil  evaporates. 
This  means  that  some  of  the  molecules  of  the  oil  leave 
the  drops  of  oil  found  upon  the  petals,  and  do  not  again 
return  to  the  flower.  These  molecules  of  oil  move  away 
in  all  directions,  and  strike  against  any  bodies  obstruct- 
ing their  path.  The  molecules  may  come  in  contact  with 
most  parts  of  our  body  without  our  being  aware  of  the 
fact.  Within  the  nose,  however,  are  nerves  especially 
adapted  to  perceive  the  presence  of  even  the  very  minute 
quantities  of  oil  which  enter  the  nostrils.  When  these 
nerves  are  bombarded  by  the  molecules  of  oil  coming 
from  the  petals  of  the  rose,  we  are  aware  of  a  sensation  in 
the  nose,  and  say — we  smell  the  rose. 

This  fragrant  oil  is  of  considerable  value  in  the  manu- 
facture of  perfumery.  About  2,700  pounds  of  roses  are 
required  to  obtain  .9  pound  of  oil,  and  this  weight  of 
roses  may  be  raised  on  an  area  of  about  1  acre.  The 
odor  of  pure  rose-oil  is  peculiarly  honey-like,  and  too 
intense  to  be  agreeable.  Its  entire  deliciousness  is  de- 
veloped only  by  a  strong  dilution  with  water,  alcohol,  or 
other  substances. 

77.  The  Small  Size  of  Molecules  shown  by  the  Sense  of 
Smell.  —  Molecules  of  substances  having  disagreeable 
odors  often  travel  rapidly,  and  a  small  quantity  is  some- 
times sufficient  to  cause  discomfort  to  all  the  persons  in  a 
large  room. 

A  very  small  particle  of  musk,  -^-5-  of  an  ounce,  will 
keep  a  room  scented  for  many  years.  The  number  of 
molecules  which  leave  the  slowly  evaporating  musk  must 
be  enormous.  Nevertheless,  the  total  loss  in  weight  in 
several  years  is  so  small  that  it  cannot  be  determined 
accurately  even  by  means  of  a  delicate  balance. 


90  ELEMENTAKY   PHYSICS 

Dogs  have  been  known  to  track  men  several  days  after 
their  feet  have  left  imprints  along-  their  line  of  wander- 
ing. Some  particles  characteristic  of  the  man  or  of  his 
clothing  must  have  been  left  behind  in  his  tracks.  The 
total  quantity  of  the  particles  must  be  very  small,  since 
they  cannot  be  detected  by  means  of  a  microscope. 
Nevertheless,  the  total  number  of  molecules  left  behind 
in  each  imprint  must  be  sufficient  to  enable  the  dog  to 
recognize  their  presence,  even  one  or  two  days  after  the 
tracks  were  made.  Moreover,  this  is  true  notwithstand- 
ing the  fact  that,  in  the  meantime,  many  molecules  must 
have  passed  away  from  the  imprints.  Molecules  must, 
therefore,  be  very  small  (§§  65,  66,  81). 

78.  Molecules  of  Solids   May  Enter  between  the  Mole- 
cules of  a  Liquid. — When  sugar  is  placed  in  water,  it  dis- 
appears.    The  sugar  is  said   to   dissolve   in  the  water. 
It  has  not  ceased  to  exist.    If  the  water  be  removed  by 
evaporation,  the   sugar  once  more  may  be    recovered. 
When   sugar  dissolves,   the    molecules  of    sugar  sepa- 
rate and  enter    the    spaces  between    the  molecules   of 
water.     The  individual  molecules  of  sugar  are  so  small 
that,  when  they  have  once  become  separated,  they  can- 
not be  recognized  even  under  the  microscope.     When 
copper  sulphate  is  dissolved  in  water,  the  individual 
molecules  separate  also,  and  can  no  longer  be  recognized 
by  means  of  the  microscope.     But  they  have  not  ceased 
to  exist.     Even  the  color  of  the  individual  molecules  is 
retained,  for  every  part  of  the  water  in  which  the  copper 
sulphate  is  dissolved  shows  the  blue  color. 

79.  The  Cause   of  Solution. — What  causes  the  sugar  to 
dissolve  in  water  I     A  difference  in  the  relative  strength  of 
certain  molecular  forces.     One  of  these  forces  is  the  mu- 
tual attraction  of  the  molecules  of  sugar  for  one   another, 


MOLECULAR  PHENOMENA  91 

which  causes  them  to  remain  together  and  form  a  lump. 
The  second  force  is  the  mutual  attraction  between  the 
molecules  of  water.  The  third  force  is  the  mutual  attrac- 
tion between  sugar  and  water. 

The  cohesion  between  the  molecules  of  sugar  is  much 
greater  than  that  between  the  molecules  of  water.  This 
is  shown  by  the  fact  that  the  point  of  a  knife  can  be 
thrust  much  more  readily  into  water  than  into  a  lump  of 
sugar.  In  order  to  separate  the  molecules  of  sugar, 
a  third  force  must  exist  which  is  stronger  than  either 
the  cohesion  of  sugar  or  the  cohesion  of  water.  This 
third  force  is  the  adhesion  between  the  molecules  of 
sugar  and  those  of  water. 

The  mutual  attraction  between  sugar  and  water  is  so 
great  that  both  the  molecules  of  water  and  the  molecules 
of  sugar  are  drawn  apart.  Sugar  molecules  draw  apart 
water  molecules,  and  water  molecules  draw  apart  sugar 
molecules,  until  finally  the  same  proportion  of  sugar  is 
found  in  every  part  of  the  water.  The  solution  has  be- 
come uniform. 

80.  The  Small  Size  of  Molecules  Shown  by  Solutions. — 
When  sugar  and  copper  sulphate  dissolve  in  water,  their 
molecules  continue  to  separate  until  no  particles  of  sugar 
or  copper  sulphate  can  be  recognized  even  under  the 
most  powerful  microscope.  Since  the  smallest  particles 
which  can  be  detected  by  means  of  the  best  microscopes 
have  a  diameter  of  about  ^QQQ  of  an  inch,  molecules  of 
sugar  and  of  copper  sulphate  are  smaller  than  1U01OUO  of  an 
inch. 

When  T^07}-  of  an  ounce  of  indigo  is  dissolved  in  sul- 
phuric acid,  it  gives  a  distinctly  blue  color  to  one  cu- 
bic foot  of  water.  This  is  equivalent  to  440  0^()  00()  of  an 
ounce  of  indigo  to  a  drop  of  water.  Nevertheless,  each 


92  ELEMENTARY  PHYSICS 

drop  of  water  is  evenly  tinged  with  blue.  In  order  that 
such  a  small  quantity  of  indigo  may  give  an  even  tinge 
of  blue  to  each  drop  of  water,  the  number  of  molecules  in 
even  440,000,000  °f  an  ounce  of  indigo  must  be  large.  In  order 
that  many  molecules  may  exist  in  44U>OUO>OUO  of  an  ounce,  the 
molecules  must  be  exceedingly  small  (§§  65,  66,  78). 

81.  The  Sense  of  Taste. — Absolutely  insoluble  materials 
are  tasteless.    If  the  substances  are  not  already  dissolved 
in  some  liquid,  in  order  to  be  tasted  they  must  be  dis- 
solved in  the  saliva,  as  soon  as  they  are  placed  within  the 
mouth.     Those  substances  which  are  most  readily  dis- 
solved are  most  easily  tasted.     Sugar  and  salt  are  good 
examples.     In  the  juices  of  many  fruits,  sugar  and  other 
substances  are  already   dissolved.    It  is  owing  to  the 
presence  of  the  dissolved  sugar  that  the  tastes  of  many 
fruits  are  so  agreeable. 

The  sense  of  taste  is  caused  by  the  motion  of  the  dis- 
solved molecules.  Located  in  different  parts  of  the 
mouth,  especially  upon  the  tongue,  are  certain  nerves 
which  are  very  susceptible  to  the  irritations  caused 
by  the  motions  of  some  kinds  of  molecules,  while  indif- 
ferent to  the  motions  of  other  kinds.  When  the  mole- 
cules of  many  of  the  dissolved  substances  come  in  contact 
with  these  nerves,  a  sensation  is  produced  known  as  the 
sensation  of  taste.  The  nerves  are  called  the  nerves  ,of 
taste. 

82.  The  Amount  of  a  Substance  Which  Can  Be  Dissolved  by 
a  Liquid  Depends  upon  the  Temperature  of  the  Liquid,— The 
quantity  of  a  solid  which  will  dissolve  in  a  liquid  de- 
pends in  some  manner  upon  the  distance  between  the 
molecules  of  the  liquid.     As  the  molecules  of  the  liquid 
move  farther  apart,  the  space  available  for  the  molecules 
of  the  solid  increases,  and  a  greater  amount  of  the  solid 


MOLECULAR   PHENOMENA  93 

can  be  dissolved.  The  distance  between  the  molecules 
of  any  substance  depends  upon  its  temperature,  and  in- 
creases (with  few  exceptions),  as  the  temperature  in- 
creases. The  amount  of  a  solid  which  can  be  dissolved 
in  a  liquid  depends,  therefore,  upon  the  temperature  of 
the  liquid. 

Water  at  150  degrees  Fahrenheit  will  dissolve  more 
sugar  than  the  same  quantity  of  water  at  100  degrees 
Fahrenheit.  When  sugar  is  placed  in  water  already 
saturated  with  sugar,  the  sugar  last  added  falls  to  the 
bottom  of  the  solution  and  remains  there  undissolved. 

Corresponding  to  each  degree  of  temperature  there  is  a 
definite  distance  between  the  molecules  of  the  liquid,  and 
a  definite  quantity  of  the  solid  which  can  be  dissolved. 
When  at  this  temperature  the  liquid  has  dissolved  all  the 
material  which  it  can  hold,  it  is  said  to  be  a  saturated 
solution.  This  solution  will  be  a  saturated  solution  as 
Jong  as  its  temperature  remains  constant.  Raising  the 
temperature  of  a  saturated  solution,  therefore,  places  it 
in  an  unsaturated  condition,  for  if  its  temperature  be 
raised,  the  liquid  can  dissolve  more  of  the  solid. 

83.  The  Cooling  of  a  Saturated  Solution  Causes  the  Ex- 
clusion  of  a  Part  of  the  Dissolved  Solid. — If  a  saturated 
solution  is  cooled,   the  spaces  between    the    molecules 
of  the  liquid  decrease,  all  of  the  solid  can  no  longer  be 
held  in  solution,  and  a  part  is  crowded  out  of  the  liquid. 
The   quantity  of  the  solid  excluded  depends  upon  the 
amount  of  fallin  temperature  of  the  liquid.     The  forced- 
out  molecules  of  the  solid  settle  upon  the  sides  and  bot- 
tom of  the  vessel  holding  the  solution. 

84.  Evaporation  Causes  an  Exclusion  of  Fart  of  the  Dis- 
solved Material, — In  a  saturated  solution  all  the  spaces 
available  between  the  molecules  of  the  liquid  are  occu- 


94  ELEMENTARY   PHYSICS 

pied  by  the  molecules  of  the  dissolved  solid.  When  a 
part  of  the  liquid  evaporates,  the  molecules  of  the  solid 
which  were  between  the  molecules  of  that  portion  of  the 
liquid  which  has  evaporated  cannot  pass  into  the  spaces 
between  the  molecules  of  the  liquid  which  is  left  in  the 
vessel,  because  the  spaces  are  already  filled.  These 
molecules  then  settle  upon  the  sides  and  the  bottom  of 
the  vessel.  As  the  liquid  continues  to  evaporate,  the 
crust  of  the  solid  material  upon  the  walls  of  the  vessel 
increases  in  thickness.  If  all  of  the  liquid  evaporates, 
all  of  the  solid  which  was  in  solution  is  left  behind  in 
the  vessel. 

The  evaporation  may  be  brought  about  either  by  sub- 
jecting the  solution  to  heat,  or  by  exposing-  it  to  air  at 
ordinary  temperatures.  In  the  first  case,  evaporation  will 
be  rapid.  In  the  second  case,  it  will  be  much  slower. 

85.  Crystallization.  —  When  molecules  are  slowly  ex- 
cluded from  a  saturated  solution,  either  by  very  slow 
cooling-  or  by  evaporation,  the  molecules  often  do  not 
form  a  crust  of  even  thickness  on  the  walls  of  the  vessel, 
but  collect  in  groups.  If,  before  cooling  or  evaporation 
is  begun,  a  small  fragment  of  the  solid  is  lowered  into 
the  liquid  by  means  of  a  string,  the  excluded  molecules 
seem  to  collect  by  preference  upon  this  fragment.  In 
this  manner  groups  of  molecules  of  unusual  size  may  be 
secured  at  the  end  of  a  string,  and  the  quantity  of  mole- 
cules settling  upon  the  sides  and  the  bottom  of  the  vessel 
may  be  considerably  reduced. 

If  these  groups  of  molecules  be  examined  very  care- 
fully, it  will  be  discovered  that  they  consist  of  a  combina- 
tion of  smaller  groups  called  crystals.  The  surface  of 
each  crystal  is  made  up  of  a  number  of  flat  smooth  faces 
which  join  each  other  along  straight  edges.  Crystals  of 


MOLECULAR  PHENOMENA  95 

the  same  substance  formed  in  the  same  solution  usually 
resemble  each  other  very  much  in  form,  although  their 
faces  are  usually  not  all  of  the  same  form  and  size.  Even 
when  the  bases  of  these  crystals  are  grown  together  so 
that  only  the  free  surfaces  of  the  crystals  can  be  exam- 
ined, their  similarity  in  form  may  be  recognized.  In 
order  to  make  evident  their  similarity  in  form,  crystals 
must  be  placed  so  that  the  corresponding  faces  of  the 
different  crystals  which  are  inclined  toward  one  another 
at  the  same  angles  occupy  the  same  relative  positions. 

86.  Solids  maybe  Recognized  by  the  Form  of  their  Crystals. 
— A  careful  study  of  the  crystals  of  different  kinds  of  sol- 
ids shows  that  crystals  composed  of  the  same  kinds  of 


Salt  Quartz  Alum  Sugar 


molecules  are  constructed  on  the  same  general  plan.  This 
is  usually  easily  recognized  when  the  crystal  faces  are 
not  large.  On  the  other  hand,  when  crystals  composed  of 
one  kind  of  molecules  are  compared  with  crystals  com- 
posed of  another  kind,  it  is  noticed  that  the  crystals  dif- 
fering in  composition  are  usually  dissimilar  in  form 
(Fig.  61).  It  is  possible,  therefore,  to  recognize  sub- 
stances by  the  form  of  their  crystals.  It  is  not  very 
likely  that  this  can  be  done  readily  and  with  confidence 
by  beginners,  even  in  the  case  of  crystals  of  very  common 
substances.  But  an  examination  of  a  few  crystals,  such 
as  those  of  salt,  quartz,  and  alum,  will  give  a  fair  idea  of 
the  principle  involved. 

Crystals  of  salt  have  six  principal  rectangular  faces,  ar- 


96  ELEMENTARY  PHYSICS 

ranged  like  the  faces  of  a  cube.  All  the  faces,  therefore, 
form  angles  of  90  degrees  with  the  immediately  adjoining 
faces. 

Crystals  of  quartz  have  the  form  of  a  six-sided  prism, 
terminated  at  each  end  (if  both  ends  are  free)  with  a  six- 
sided  pyramid.  The  faces  of  the  prism  meet  each  other 
at  angles  of  120  degrees.  The  faces  of  the  prism  meet  the 
faces  of  the  pyramid  at  angles  of  about  140  degrees.  The 
alternate  faces  of  the  pyramid  are  sometimes  slightly 
rougher  in  appearance  than  the  intermediate  faces. 

Crystals  of  alum  are  formed  by  a  combination  of  two 
geometric  forms,  the  octohedron  and  the  cube.  The 
typical  octohedron  consists  of  two  similar  pyramids,  fac- 
ing in  opposite  directions  and  united  along  their  base. 
Each  pyramid  consists  of  four  triangular  faces.  All  the 
faces  are  identical  in  form  ;  they  are  equilateral  triangles. 
The  faces  of  the  octohedron  meet  along  straight  edges  at 
angles  of  about  110  degrees.  The  cube  consists  of  six 
rectangular  faces  meeting  each  other  at  angles  of  90  de- 
grees. In  the  combination,  the  faces  of  the  cube  usually 
appear  like  little  faces  cutting  off  the  corners  of  the  octo- 
hedron. The  faces  of  the  cube  and  those  of  the  octohe- 
dron meet  at  angles  of  about  125  degrees. 

Crystals  of  sugar  are  more  complex  in  form.  While 
not  readily  described  in  language  which  those  not  famil- 
iar with  geometry  will  easily  understand,  the  fact  that 
they  have  a  characteristic,  form  of  their  own  (monoclinic) 
is  readily  recognized. 

87.  Crystals  Suggest  that  the  Molecules  of  Different  Sub- 
stances Differ  in  Form — No  matter  whether  large  or 
small,  whether  single  or  in  groups,  whether  presenting 
their  natural  color  or  giving  evidence  of  being  stained 
by  foreign  substances,  crystals  composed  of  the  same 


MOLECULAR  PHENOMENA  97 

kinds  of  molecules  possess  the  same  general  form.  This 
shows  that  they  have  been  developed  in  accordance  with 
the  same  general  plan. 

It  is  difficult  to  understand  how  this  can  be  possible,  if 
all  molecules  of  the  same  kind  do  not  follow  the  same 
relative  arrangement  in  all  crystals.  The  remarkable 
uniformity  of  the  arrangement  of  the  molecules  is  shown 
in  a  striking 'way  by  the  wonderfully  even  surface  of  the 
flat  faces  of  the  crystals.  Few  objects  in  nature  are 
as  absolutely  smooth  as  are  many  of  the  faces  on  the 
smaller-sized  crystals  of  most  minerals.  The  absolute 
identity  of  the  angle  made  by  corresponding  faces  of 
crystals  composed  of  the  same  kind  of  molecules  further 
indicates  the  absolute  identity  in  the  arrangement  of  the 
molecules.  How  is  this  arrangement  possible,  if  mole- 
cules  of  the  same  substance  are  not  exactly  alike  in  size 
and  form  ? 

The  reason  why  crystals  of  different  substances  differ 
in  form  must  be  because  the  molecules  of  different  sub- 
stances also  differ  in  size  and  form. 

88.  Large  Crystals  Produced  by  Slow  Cooling  or  Evapo- 
ration.— Molecules  excluded  from  a  saturated  solution  by 
cooling  or  evaporation  do  not  settle  with  equal  readiness 
upon  all  objects  with  which  they  come  in  contact.  They 
seem  to  display  a  kind  of  preference  for  certain  loca- 
tions. For  instance,  they  are  more  apt  to  settle  upon 
rough  bodies,  such  as  strings  and  pieces  of  wood,  than 
upon  smooth  objects.  But  the  greatest  preference  seems 
to  be  shown  for  crystals  composed  of  molecules  identical 
in  kind  with* those  which  are  seeking  a  location.  For 
this  reason  the  crystals  first  formed  usually  grow  much 
more  rapidly  than  those  which  come  into  existence  at  a 
later  time.  Even  fragments  of  crystals  lowered  into  the 


08  ELEMENTARY  PHYSICS 

solution  by  means  of  strings  form  good  collecting  cen 
tres,  if  they  consist  of  the  same  kind  of  molecules  as 
those  which  are  being  excluded  from  the  liquid.  By 
their  use  the  likelihood  is  much  increased  of  securing 
large  crystals  which  may  be  removed  easily  from  the 
vessel. 

In  order  to  secure  large  crystals  of  salt,  dissolve  a  con- 
siderable quantity  of  salt  in  water,  until  the  salt  last 
added  remains  undissolved  even  after  considerable  stir- 
ring. Set  aside  the  vessel  containing  the  solution,  in  a 
place  where  evaporation  will  take  place  very  slowly.  As 
the  water  gradually  evaporates,  the  crystals  will  develop. 
In  several  weeks  fine  large  crystals  may  be  obtained. 

In  order  to  secure  large  crystals  of  alum  or  sugar,  dis- 
solve as  much  of  the  solid  as  possible  in  hot  water.  Tie 
a  piece  of  the  solid  at  the  end  of  a  string  and  lower  it 
into  the  liquid.  Then  allow  the  saturated  solution  to 
cool  very  slowly.  In  the  case  of  sugar  the  temperature 
of  the  heated  water  should  not  be  raised  above  140  de- 
grees Fahrenheit  and,  while  crystallization  is  taking 
place,  the  temperature  should  never  be  allowed  to  fall 
below  70  degrees  Fahrenheit. 

Good  crystals  of  quartz  can  easily  be  purchased  from 
any  dealer  in  minerals.  (A.  E.  Foote,  Philadelphia.) 

89.  Small  Crystals  Produced  by  Rapid  Cooling  or  Evapo- 
ration.— In  the  formation  of  crystals  of  large  size,  the 
rate  of  cooling  seems  to  be  a  very  important  element.  It 
takes  time  for  molecules  to  travel  through  any  liquid. 
If  there  be  ample  time,  most  of  the  excluded  molecules 
find  their  way  to  the  crystals  first  formed,  so  that  the 
first  formed  crystals  show  the  greatest  growth.  If,  how- 
ever, the  solution  be  cooled  rapidly,  the  molecules  of  the 
solid  in  solution  are  excluded  so  rapidly  and  in  such 


MOLECULAR  PHENOMENA  99 

great  numbers  that,  before  they  can  reach  the  crystals 
already  in  existence,  they  form  new  groups  at  many 
points  within  the  liquid.  These  groups  form  new  cen- 
tres of  collection,  so  that,  in  case  of  rapid  cooling,  crys- 
tals are  formed  simultaneously  throughout  the  liquid. 
These  crystals  may  at  first  be  so  small  that  they  can  be 
recognized  only  by  means  of  the  microscope.  If  they  are 
very  numerous,  the  solution  simply  assumes  to  the  naked 
eye  a  turbid  appearance. 

Granulated  sugar  and  the  finer  grades  of  table  salt  are 
produced  by  rapid  evaporation  accompanied  by  continual 
stirring,  in  order  that  the  small  crystals,  which  are  in 
process  of  formation  throughout  the  liquid,  may  not 
grow  together.  When  the  crystals  are  of  proper  size 
the  liquid  is  withdrawn  or  the  crystals  are  removed. 
The  rate  of  cooling  may  be  so  rapid  that  the  molecules 
cannot  find  time  to  arrange  themselves  in  an  orderly 
manner.  They  then  form  upon  the  walls  and  the  bottom 
of  the  vessel  a  coating  which  gives  no  evidence  of  crys- 
tallization. 

The  lavas  which  have  poured  out  from  the  volcanoes  in 
different  parts  of  the  world  have  cooled  with  such  vary- 
ing degrees  of  rapidity  that  some  of  them  have  turned 
into  a  mass  of  crystals,  while  others  show  no  more  than 
the  faintest  trace  of  crystallization.  Under  the  micro- 
scope all  gradations  may  be  seen  between  lavas  com- 
posed entirely  of  crystals,  and  lavas  which  are  practically 
without  trace  of  crystallization. 


CHAPTEE  in 

HEAT 

90.  The  Three  States  of  Matter. — It  is  a  familiar  fact  that 
water  may  exist  under  three  different  forms.  It  may  be 
a  solid,  a  liquid,  or  a  gas.  When  solid,  it  is  called  ice, 
when  liquid,  it  is  called  water,  and  when  gaseous,  it  is 
called  steam.  The  word  water  may  also  be  used,  in  a 
general  sense,  so  as  to  include  all  forms  of  water.  In 
this  case  ice  is  looked  upon  as  solid  water,  and  steam  as 
water  in  the  gaseous  state. 

It  is  not  so  well  known  that  many  other  substances 
also  exist  in  three  forms.  The  chief  reason  for  this  lack 
of  information  is  the  fact  that  few  substances  are  as  use- 
ful as  water,  in  all  three  forms  in  which  they  can  exist. 
Few  substances,  therefore,  have  been  studied  as  carefully 
as  water.  Paraffine,  sealing-wax,  lead,  and  iron  are  all 
solids  at  ordinary  temperatures,  but  when  paraffine  is 
used  to  form  thin  layers  over  jelly  so  as  to  exclude  the 
air,  and  when  sealing-wax  or  lead  is  employed  to  seal 
fruit  jars,  these  substances  are  first  melted.  When  it  is  de- 
sired to  cast  lead  into  the  form  of  bullets  and  iron  into  the 
form  of  stoves,  they  must  first  be  liquefied  and  then  poured 
into  moulds.  It  is,  therefore,  well  known  that  these  sub- 
stances exist  both  as  solids  and  as  liquids,  but  it  is  not 
so  well  known  that  some  of  these  can  also  exist  as  gases. 
Nevertheless,  paraffine  can  easily  be  heated  sufficiently 

100 


HEAT 


to  be  turned  into  a  gaseous  form,  and,  in  the  enormously 
heated  atmosphere  of  the  sun,  lead  and  iron  exist  in  the 
form  of  gases. 

Another  reason  why  the  existence  of  many  substances 
in  all  three  forms  is  unknown,  is  because  in  many  sub- 
stances some  of  these  forms  require  extremely  high  or 
extremely  low  temperatures.  Most  metals,  for  instance, 
assume  the  gaseous  form  only  at  very  high  temperatures^ 
and  many  of  the  more  common  gases  become  liquid  and 
solid  only  at  very  low  temperatures.  For  this  reason, 
air,  which  is  composed  almost  wholly  of  two  gases,  one 
part  of  oxygen  and  four  parts  of  nitrogen,  has  been 
known  in  the  liquid,  and  even  in  the  solid  state,  only 
within  the  last  twenty-five  years. 

As  far  as  known  all  substances  when  sufficiently  cooled, 
assume  the  solid  state.  Some  substances  change  from 
gases  into  solids  or  from  solids  into  gases,  directly,  with- 
out assuming  the  intermediate  liquid  state  (§  75).  Many 
solids,  when  heated  beyond  a  certain  temperature,  do 
not  turn  into  liquids,  but  change  their  nature  entirely. 
Wood,  for  instance,  when  heated  beyond  a  certain  tem- 
perature, does  not  become  liquid,  but  burns  ;  it  changes 
into  ashes  and'certain  kinds  of  gas.  Changes  of  this  kind 
will  be  more  fully  studied  later  (§  195). 

The  general  fact  remains,  that  there  are  three  states  of 
matter.  The  changes  of  matter  from  one  state  into  an- 
other are  accompanied  by  interesting  phenomena.  The 
phenomena  will  be  studied  in  the  following  paragraphs, 
water  being  taken  as  a  familiar  example. 

91.  The  Temperature  of  Ice  Is  Variable.  —  The  tempera- 
ture of  a  piece  of  iron  exposed  to  the  open  air  varies  with 
that  of  the  surrounding  atmosphere.  In  summer  it  is 
warm,  and  in  winter  it  is  cold. 


PHYSICS 

In  a  similar  manner  the  temperature  of  a  piece  of  ice 
varies  with  the  surrounding-  atmosphere.  At  Werkojank 
in  Siberia  the  temperature  of  the  air  has  been  known  to 
fall  to  90°  below  zero,  Fahrenheit,  or  122°  below  the 
freezing-  point.  This  is  the  coldest  region  so  far  dis- 
covered on  our  globe.  The  temperature  of  the  ice  ex- 
posed  to  the  air  in  this  locality  must  also  fall  to  90°  below 
zero. 

In  Ohio,  temperatures  of  15°  below  zero,  or  47°  below 
the  freezing  point,  are  not  frequent,  but  occur  often 
enough  to  be  within  the  experience  of  anyone  who  has 
lived  within  the  State  ten  or  more  years.  During  the 
existence  of  such  cold  weather,  the  temperature  of  ice 
also  falls  to  15°  below  zero.  Place  the  thermometer  in 
snow  on  some  day  when  the  temperature  of  the  air  is 
much  below  the  freezing  point  of  water.  When  the 
weather  becomes  warmer,  the  temperature  of  ice  rises, 
and  when  the  temperature  of  the  air  exceeds  32°  above 
zero,  the  ice  melts. 

Ice-cream,  at  a  temperature  of  32°  F.,  will  melt  readily 
in  summer  time.  But  if  its  temperature  be  reduced  con- 
siderably below  32°  F.,  it  will  remain  in  a  frozen  condition 
until  its  temperature  has  risen  to  32°  F.,  when  the  process 
of  melting  begins.  For  this  reason  caterers  sometimes 
lower  the  temperature  of  icercream  which  is  intended 
for  evening  parties,  so  much  that  it  may  become  a  little 
warmer  and  still  be  in  fit  condition  to  eat  several  hours 
after  it  has  been  delivered  (§  114). 

92.  The  Temperatures  of  Water  and  Steam  are  Variable. — 
That  the  temperature  of  water  varies  considerably  is  too 
familiar  a  fact  to  require  special  mention.  Between  the 
freezing  point,  32°  F.,  and  the  boiling  point,  212°  F.,  there 
is  a  difference  of  180  degrees. 


HEAT  103 

It  is  not  so  well  known,  however,  that  the  temperature 
of  steam  may  also  vary  considerably.  At  ordinary  press- 
ures of  the  atmosphere,  at  sea  level,  water  is  converted 
into  steam  soon  after  its  temperature  reaches  212°  F. 
The  steam  which  is  formed  also  has  a  temperature  of 
212°  F.  If  the  steam  is  placed  in  a  closed  vessel,  its  tem- 
perature may  easily  be  raised  considerably  above  212  F. 
The  steam  generated  in  the  boiler  of  an  engine  always 
rises  above  this  temperature.  When  the  temperature  of 
steam  is  so  great  that  no  water  is  present  in  a  liquid 
condition,  even  in  the  form  of  a  mist,  the  steam  is  said  to 
be  dry. 

93.  The  Temperature  at  which  Liquefaction  and  Solidifica- 
tion Take  Place. — At  ordinary  pressures  of  the  air,  ice 
melts  at  a  temperature  of  32°  F.  This  temperature  is 
known  as  the  melting  point  of  ice.  Other  substances  do 
not  melt  at  this  temperature  but  have  their  own  char- 
acteristic melting  points.  Mercury,  for  instance,  melts  at 
40°  below  zero,  Fahrenheit,  paraffine  melts  at  129°  above 
zero,  sulphur  at  239°,  tin  at  449°,  lead  at  619°,  zinc  at 
773°,  silver  at  1749°,  copper  at  1930°,  hard  glass  at  2012°, 
nickel  at  2642°,  and  iron  at  2912°  F. 

Many  substances  melt  at  temperatures  so  low  that  they 
are  never  seen  in  a  solid  state  except  in  laboratories 
where  very  low  temperatures  are  produced  artificially. 
This  is  especially  true  of  many  gases,  which  are  not  only 
never  seen  as  solids  outside  of  laboratories,  but  which 
are  never  seen  even  as  liquids,  excepting  under  similar 
conditions. 

The  fact  that  different  substances  melt  at  different  de- 
grees of  temperature  is  utilized  in  a  number  of  ways. 
Lead,  zinc,  copper,  and  glass,  for  instance,  may  be  read- 
ily melted  in  a  vessel  constructed  of  iron.  Why  ? 


104  ELEMENTARY  PHYSICS 

If  water  is  cooled  sufficiently  it  freezes.  The  tempera- 
ture at  which  it  freezes  is  the  same  as  that  at  which  it 
melts.  In  other  words,  the  point  of  liquefaction  is  also 
the  point  of  solidification.  This  is  also  true  of  other 
substances. 

94.  The  Temperature  at  which  Ebullition  and  Condensa- 
tion Take  Place. — When  subjected  to  a  pressure  of  one 
atmosphere,  the  ordinary  pressure  of  the  air  at  sea  level, 
water  boils  at  a  temperature  of  212°  F.     This  tempera- 
ture is  known    as  the   boiling  point    of    water.      Other 
liquids  do  not  boil  at  this  temperature  but  have  their 
own  characteristic  boiling  point.     Under  a  pressure  of 
one  atmosphere,  ether  boils  at  95°,  alcohol  at  172°,  tur- 
pentine at  319°,  and  mercury  at  662°  F. 

Some  substances  boil  at  such  low  temperatures  that 
they  are  never  seen  in  the  liquid  state  except  in  laborato- 
ries. They  are  usually  known  only  in  the  form  of  gases. 
This  is  true  of  hydrogen,  nitrogen,  oxygen,  and  carbon 
dioxide. 

The  fact  that  different  liquids  boil  at  different  temper- 
atures is  utilized  in  the  separation  of  various  substances 
by  means  of  distillation.  If  a  mixture  of  water  and  alco- 
hol is  heated  above  the  boiling  point  of  alcohol  and  be- 
low that  of  water,  the  alcohol  passes  off  in  the  form  of  a 
vapor,  while  the  water  will  be  left  behind.  If  the  alco- 
hol vapor  is  passed  through  a  coil  of  tin  or  copper  pipe 
placed  in  cold  water,  the  alcohol  vapor  is  chilled  and 
returns  to  the  liquid  state.  It  may  thus  be  secured 
nearly  free  from  water. 

95.  Change  in  Volume  during  Solidification  and  Liquefac- 
tion.— The  passage  of  any  substance  from  a  liquid  to  a 
solid  state  is  invariably  accompanied  by  a  change  of 
volume. 


Volume  ofutater  before  freezing. 


HEAT  105 

Water  expands  on  turning  into  ice.    It  increases  abou-t 
^  in  volume  (Fig.  62).     The  result  is  that  when  water 
freezes  in  a  pipe  which  has  no 
outlet,  it  often  bursts  the  pipe. 
The  water  in  a  pitcher  freezes    |       roiu^^^/y^.       ~~| 
at  the  surface  first.    The  layer 
of  ice  thus  formed  serves  as  a 

huge  stopper,  preventing  the  escape  of  the  water  beneath. 
If  any  considerable  part  of  the  water  in  the  lower  part  of 
the  pitcher  freezes,  the  pitcher  is  likely  to  break.  Since 
water  expands  on  turning  into  ice,  ice  is  lighter  than 
water,  and  floats  on  the  water.  Beneath  the  layer  of  ice 
covering  the  river  during  winter,  the  water  still  has  a 
temperature  of  32°  F.,  or  slightly  above.  At  this  tem- 
perature the  kinds  of  fish  found  in  northern  climates  can 
live. 

Most  liquids  contract  on  assuming  the  solid  state. 
Water  is  one  of  the  most  notable  exceptions.  Owing  to 
the  abundance  of  water,  and  its  great  importance,  both  in 
maintaining  life  and  in  making  possible  many  lines  of 
manufacture,  the  exceptional  behavior  of  water  is  more 
familiar  than  the  general  behavior  of  other  liquids. 

When  solids  pass  into  the  liquid  state  their  change  in 
volume  is  exactly  the  reverse  of  the  change  which  takes 
place  during  solidification. 

96.  Changes  in  Volume  during  Ebullition  and  Condensa- 
tion.— The  passage  from  a  liquid  to  a  gaseous  state  is 
invariably  accompanied  by  an  increase  of  volume. 

When  water  at  a  temperature  of  212°  F.  is  changed  to 
steam  at  212°  F.,  it  does  not  change  in  temperature,  but 
it  does  change  its  state,  and  this  change  is  accompanied 
by  an  enormous  change  in  volume.  A  cubic  inch  of 
water  at  212°  is  converted  into  1696  cubic  ipches,  or 


106 


ELEMENTARY   PHYSICS 


Volume  of  water 
in  form  of  Steam 


FIG. 


nearly  one  cubic  foot,  of  steam  having  the  same  tempera- 
ture (Fig1.  63).     Water  furnishes,  bulk  for  bulk,  a  greater 

volume  of  gaseous  material  than 

O  Volume  of  water  ,-,  •, .          .  -.        .-p-i    . 

in  form  of  liquid  any  other  liquid.   This  enormous 

expansion  is  accompanied  by 
great  force,  and  is  utilized  in  all 
the  numerous  kinds  of  steam-en- 
gines. 

The  return  from  a  gaseous  to 
a  liquid  state  is  always  accom- 
panied by  a  considerable  de- 
crease in  volume. 

97.  For  the  Same  Increase  of 
Temperature,  Gases  Expand  More 
than  Liquids,  and  Liquids  Expand 
More  than  Solids. — A  considerable  number  of  well-known 
substances  remain  in  the  solid  state  when  raised  to  a 
temperature  of  70°  F.  A  large  number  are  always  in  the 
liquid  state,  and  a  much  smaller  number  are  always  in 
the  gaseous  state,  at  this  temperature. 

If  now  equal  volumes  of  a  variety  of  substances  are 
chosen,  some  of  which  are  in  the  solid,  and  others  in  the 
liquid,  or  gaseous,  states  at  70°  F.,  and  if  their  tempera- 
ture is  raised  from  70°  to  71°  F.,  it  is  found  that  all  in- 
crease in  volume,  but  that  the  amount  of  increase  is  not 
the  same  in  all  cases.  All  gases  expand  at  the  same  rate 
for  the  same  increase  of  temperature,  but  the  rate  of  ex- 
pansion of  different  liquids  .and  solids  varies  consider- 
ably. In  a  general  way  it  may  be  stated  that  for  the 
same  increase  in  temperature,  liquids  expand  on  the 
average  about  20  times,  and  gases  about  70  times  as 
much  as  solids.  The  figures  secured  will  naturally  vary 
with  the  solids  and  liquids  chosen  for  comparison. 


HEAT  107 

This  fact  is  true  also  of  different  states  of  the  same 
substance.  For  instance,  the  amount  of  increase  in  vol- 
ume of  water  for  a  rise  of  one  degree  in  temperature  is 
greater  for  water  in  the  gaseous  state  than  for  water  in  the 
liquid  state,  and  for  water  in  the  liquid  state  than  for  water 
in  the  solid  state.  These  changes  in  volume  without  a 
change  of  state  are,  however,  very  small  as  compared 
with  the  changes  in  volume  during  any  change  from  one 
state  to  another  (§§  95,  96). 

98.  Effect  of  Pressure  on  the  Melting  Point. — Water  ex- 
pands on  turning  into  ice.  Under  a  pressure  of  one  at- 
mosphere, water  freezes  at  a  temperature  of  32°  F.  If 
the  pressure  on  the  water  is  increased,  there  is  a  corre- 
sponding resistance  to  the  increase  of  volume,  and  water 
must  be  cooled  to  a  temperature  below  32°  F.  before  it 
will  expand  violently  enough  to  freeze.  Pressure  does 
not  assist  in  the  freezing  of  water.  It  interferes.  An 
increase  in  pressure  of  a  thousand  pounds  per  square 
inch,  lowers  the  freezing  point  of  water  about  1°  F. 

When  ice  melts,  it  decreases  in  volume.  An  increase 
of  the  pressure  exerted  on  the  ice  tends  to  make  it  oc- 
cupy less  volume.  If  the  temperature  of  the  ice  is  as 
high  as  32°  F.,  the  ice  can  occupy  considerably  less  vol- 
ume by  becoming  water.  Pressure,  therefore,  assists  in 
melting  ice. 

When  particles  of  ice  in  the  form  of  snow  are  pressed 
together,  they  melt  slightly  at  the  surface.  Immediately 
on  removing  the  pressure,  the  water  formed  turns  again 
into  ice.  The  ice  crystals  become  united  at  their  surfaces 
and  a  snowball  is  the  result.  When  the  rivers  of  ice 
called  glaciers  pass  over  rocks,  the  ice  is  pressed  against 
the  rock,  and  melts.  The  water  thus  formed  turns  into 
ice  again  immediately  on  passing  the  obstruction.  In 


108  ELEMENTARY   PHYSICS 

this  way  ice  is  able  to  flow  down  valleys  and  to  follow 
the  irregular  curves  along  the  sides  and  the  bottom  of 
the  valley  and  yet  may  arrive  at  the  foot  of  the  glacier 
in  the  form  of  compact  ice. 

Solids  which  expand  on  melting,  when  subjected  to 
pressure,  will  have  their  melting  point  raised  instead  of 
lowered. 

99.  Effect  of  Pressure  on  the  Boiling  Point. — Water  sub- 
jected to  a  pressure  of  one  atmosphere  boils  at  a  tem- 
perature of  212°  F.  When  water  at  this  temperature 
turns  into  steam  it  increases  in  volume  1696  times.  In 
order  to  turn  into  steam,  therefore,  water  must  occupy 
1696  times  as  much  space,  and  in  order  to  secure  this 
space  it  must  push  back  the  overlying  air. 

The  force  which  water  must  exert  in  order  to  expand 
into  steam  depends  upon  the  pressure  which  the  atmos- 
phere exerts  upon  the  water.  If  the  atmosphere  presses 
upon  the  water  with  greater  force,  the  water  must  expand 
with  greater  force  in  order  to  turn  into  steam.  If,  on  the 
contrary,  the  pressure  of  the  atmosphere  upon  the  sur- 
face of  the  water  is  less,  the  water  need  not  exert  so  much 
force  in  order  to  turn  into  steam. 

The  force  which  the  molecules  of  water  must  exert  in 
order  that  water  may  turn  into  steam,  must  be  suffi- 
ciently energetic  to  cause  the  molecules  to  leave  the 
water  remaining  in  the  vessel,  and  to  enable  them  to  pass 
off  into  space.  The  motions  of  molecules  which  enable 
them  to  move  about,  become  more  energetic  as  their 
temperature  increases.  Hence  in  order  that  molecules 
of  water  may  escape  in  the  form  of  steam,  their  tempera- 
ture must  be  greater  when  the  atmospheric  pressure  is 
greater,  and  their  temperature  need  not  be  so  high  when 
the  atmospheric  pressure  is  less. 


HEAT  109 

Increase  of  pressure  raises  the  boiling  point  of  a  liquid ; 
decrease  of  pressure  lowers  the  boiling-  point. 

100.  Determination  of  Elevation  by  Boiling  Point — Since 
the  pressure  exerted  by  air  varies  with  the  elevation  of 
the  object  pressed  upon  (§  13),  the  force  which  water 
must  exert  in  order  to  turn  into  steam  also  depends  upon 
the  elevation  above  sea  level.  At  a  greater  elevation  less 
force  is  required,  at  a  lower  elevation  a  greater  force  is 
necessary.  Hence  at  a  greater  elevation  water  boils  at  a 
lower  temperature,  and  at  a  lower  elevation  a  higher  tem- 
perature is  required.  If  a  table  be  prepared  giving  ac- 
curate information  on  the  relationship  between  boiling 
points,  atmospheric  pressures,  and  elevations,  the  boiling 
point  of  water  at  any  locality  may  be  used  as  a  means  of 
determining  its  elevation. 

In  a  general  way  it  may  be  stated  that  a  lowering  of  1° 
F.  in  the  boiling  point  of  water  is  equivalent  to  a  rise  in 
elevation  of  approximately  538  feet.  At  Quito,  the  high- 
est city  in  the  Andes  of  South  America,  water  boils  at  a 
temperature  of  194.2  F.  This  is  17.8°  F.  below  the  boil- 
ing point  of  water  at  sea  level.  Therefore,  the  elevation 
of  Quito  is  approximately  17.8x538=9574.4  feet.  At 
what  temperature  does  water  boil  at  the  locality  in  which 
you  live  ?  What,  therefore,  is  its  elevation  above  sea 
level  approximately  ? 

On  the  top  of  Mount  Blanc  water  boils  at  a  tempera- 
ture of  185.8°  F.  Eggs  cannot  be  cooked  in  water  boil- 
ing at  this  temperature.  Many  other  cooking  operations 
requiring  water  boiling  at  higher  temperatures  cannot 
be  carried  on  in  open  vessels  at  these  elevations. 

In  boilers  connected  with  high-pressure  engines  the 
boiling  point  is  often  raised  to  temperatures  exceeding 
300°  F. 


110  ELEMENTAEY   PHYSICS 

101.  Relation  between  Atmospheric  Pressure  and  Evapo- 
ration.— When  water  evaporates,  the  molecules  separate 
and  form  a  vapor  which  passes  off  into  the  air.     The 
greater  the  pressure  of  the  atmosphere,  the  greater  is 
the  difficulty  with  which  the  molecules  of  the  water  can 
pass  off  into  the  air.     Therefore,  evaporation  takes  place 
much  more  readily  in  localities  where  atmospheric  press- 
ure is  low. 

The  relationship  between  atmospheric  pressure  and 
evaporation  are  very  similar  to  those  between  atmos- 
pheric pressure  and  ebullition,  or  boiling. 

In  the  evaporation  of  syrups  during  the  manufacture 
of  sugar,  the  operation  is  carried  on  in  large  air-tight 
chambers  from  which  the  air  and  the  vapor  formed  by 
the  evaporation  are  removed  by  an  air-pump.  The  space 
above  the  syrup  approaches  so  nearly  to  the  conditions 
of  a  vacuum,  that  the  molecules  of  the  water  are  far  less 
impeded  in  their  efforts  to  leave  the  surface  of  the  liquid. 
Evaporation  takes  place,  therefore,  much  more  readily 
and  at  much  lower  temperatures,  when  sugar  is  heated  in 
vacuum-pans.  This  is  done  in  order  to  keep  a  tempera- 
ture sufficiently  low  so  as  not  to  burn  the  sugar. 

102.  Difference  between  Intensity  and  Quantity  of  Heat. — 
A  further  study  of  the  phenomena  involved  in  the  change 
of  a  body  from  a  solid  to  a  liquid,  or  from  a  liquid  to  a 
gaseous  state,  makes  it  desirable  to  have  an  accurate  idea 
as  to  the  quantity  of  heat  which  is  necessary  to  produce 
these  changes. 

The  conception  of  quantity  of  heat  is  not  so  familiar  to 
most  persons  as  the  conception  of  intensity  of  heat.  The 
intensity  of  heat  shown  by  a  body  is  known  as  its  tem- 
perature. The  unit  of  temperature  is  called  a  degree. 
The  position  of  the  top  of  a  column  of  mercury  at  various 


HEAT  111 

degrees  of  temperature  is  marked  off  on  the  scales  of 
thermometers,  and  by  means  of  these  instruments  the 
temperature  of  a  body  can  be  directly  measured. 

An  ounce  of  red-hot  iron  (at  a  temperature  of  about 
1000°  F.)  possesses  a  much  greater  intensity  of  heat  than 
boiling  water  at  a  temperature  of  212°.  The  intensity  of 
heat  does  not  depend  upon  the  quantity  of  material,  but 
merely  upon  the  question,  how  hot  it  is.  Therefore,  an 
ounce  of  red-hot  iron  has  much  greater  intensity  of 
heat,  notwithstanding  the  small  quantity  of  iron,  than  a 
barrel  of  boiling  water. 

Which,  however,  possesses  the  greater  quantity  of 
heat  ?  Suppose  that  on  a  cold  winter  day  the  ounce  of 
red-hot  iron  and  the  barrel  of  boiling  water  be  placed  in 
separate  rooms,  both  of  small  size  and  having  the  same 
temperature.  Which  will  heat  up  the  air  in  the  room  to 
the  higher  temperature, — the  ounce  of  red-hot  iron  or 
the  barrel  of  boiling  water  ?  Unquestionably,  the  latter. 
Or,  suppose  that  the  ounce  of  red-hot  iron  and  the  bar- 
rel of  boiling  water  be  placed  in  large  tanks  half  full  of 
snow  ?  Which  will  melt  the  most  snow  ?  Again,  un- 
questionably the  barrel  of  boiling  water.  Which,  there- 
fore, possesses  the  greater  quantity  of  heat  ?  The  barrel 
of  boiling  water. 

From  this  it  may  be  seen  that  the  quantity  of  heat  de- 
pends not  only  upon  the  temperature  of  a  body  but  also 
upon  the  quantity  of  material  contained  in  that  body. 

103.  The  Unit  of  Quantity  of  Heat. — In  order  to  com- 
pare different  quantities  of  heat,  it  is  desirable  to  have 
some  method  of  measuring  this  quantity.  When  we 
wish  to  measure  length,  we  use  a  wooden  or  steel  rule 
upon  which  feet,  inches,  and  parts  of  inches  have  been 
carefully  marked.  When  we  wish  to  measure  weight,  it 


ELEMENTARY  PHYSICS 

is  sufficient  for  ordinary  purposes  to  use  some  form  of  a 
balance  accompanied  by  a.  set  of  known  weights.  The 
intensity  of  heat  is  measured  by  means  of  a  thermome- 
ter supplied  with  a  scale  upon  which  the  degrees  are 
indicated.  But  there  is  no  instrument  to  determine 
the  quantity  of  heat.  Therefore  the  quantity  of  heat 
contained  in  a  body  must  be  determined  indirectly 
by  drawing  conclusions  from  some  of  the  effects  which 
the  heat  contained  is  able  to  produce.  In  practice,  the 
quantity  of  heat  contained  in  a  body  is  determined  by 
noticing  through  what  range  of  temperature  it  will  raise 
a  known  quantity  of  water. 

Just  as  there  must  be  definite  units  in  order  to  meas- 
ure length,  weight,  and  temperature,  so  there  must  also 
be  some  definite  unit  in  order  to  measure  quantity  of 
heat.  The  unit  here  accepted  is  the  quantity  of  heat 
which  will  raise  the  temperature  of  one  pound  of  water 
from  32°  F.  to  33°  F.  This  unit  is  called  a  calorie,. 

Within  ordinary  ranges  of  temperature  it  takes  very 
nearly  the  same  quantities  of  heat  to  raise  a  pound  of 
water  one  degree  in  temperature,  no  matter  what  be  the 
temperature  of  the  water  to  which  the  additional  heat  is 
applied.  For  this  reason  it  is  sufficiently  accurate  for 
most  purposes  to  assume  that  any  quantity  of  heat  which 
raises  one  pound  of  water  1°  F.  above  its  original  tem- 
perature is  equal  to  a  calorie.  Therefore,  to  raise  one 
pound  of  water  15  degrees  would  require  15  calories,  and 
to  raise  10  pounds  of  water  8  degrees  would  require  80 
calories. 

This  rule  applies  only  to  water  in  the  liquid  state.  To 
raise  one  pound  of  ice  one  degree  in  temperature  requires 
.504  calorie,  and  to  raise  a  pound  of  steam  through  an 
equal  range  of  temperature  requires  .4805  calorie. 


HEAT  113 

104.  Heat  Consumed  daring  Change  of  State,  from  Solid  to 
Liquid,  or  from  Liquid  to  Gas. — Heat  is  consumed  in  rais- 
ing1 a  body  from  a  lower  to  a  higher  temperature.  Heat 
is  consumed,  for  instance,  in  raising  a  pound  of  ice  from  a 
temperature  of  10°  F.  to  32°  F.,  in  raising-  the  water  from 
32°  to  212°,  and  in  raising-  the  steam  from  212°  to  250°. 
From  the  concluding-  statements  in  the  preceding-  para- 
graph it  may  be  seen  that  the  quantities  of  heat  required 
in  the  three  cases  just  mentioned  are  about  11,  180,  and 
19  calories  respectively. 

Is  any  heat  consumed  in  changing-  a  body  from  a  solid 
to  a  liquid  or  from  a  liquid  to  a  g-aseous  state  ?  If  any 
heat  is  consumed  in  any  of  these  operations,  the  ther- 
mometer does  not  indicate  it. 

If,  for  instance,  ice  be  taken  at  10°  F.  and  be  slowly 
heated  in  a  vessel  over  a  Bunsen  burner,  a  thermometer 
imbedded  in  the  ice  will  show  a  rise  in  temperature  until 
a  temperature  of  32°  F.  is  reached.  Then  for  some  time, 
notwithstanding  the  fact  that  heat  is  being  continually 
consumed  by  the  ice,  the  thermometer  will  show  no  rise 
in  temperature.  During  this  time  the  ice  is  melting. 

As  soon  as  all  the  ice  has  melted,  the  rise  in  tempera- 
ture again  begins,  and  the  mercury  in  the  thermometer 
moves  up  steadily  until  it  indicates  a  temperature  of  212°. 
Then  the  mercury  in  the  thermometer  once  more  be- 
comes stationary.  Notwithstanding  the  fact  that  heat  is 
being  consumed  by  the  water,  no  rise  in  temperature 
takes  place.  During  this  time  the  water  is  changing 
into  steam.  When  all  of  the  water  has  been  changed 
into  steam,  the  temperature  once  more  rises  provided  the 
steam  is  confined  in  a  vessel  so  that  it  cannot  escape. 
If  it  is  desired  to  raise  steam  to  very  high  temperatures, 
it  must  be  enclosed  in  very  strong  vessels,  otherwise  the 


114  ELEMENTARY  PHYSICS 

rapidly  increasing  pressure  of  the  steam  may  cause  an 
explosion. 

Since  the  temperature  of  the  ice  when  it  begins  melt- 
ing is  32 '  F.,  and  since  the  temperature  of  the  water  im- 
mediately after  melting  has  taken  place  is  also  32°,  it  is 
evident  that  the  heat  consumed  has  not  been  utilized  in 
causing  a  rise  in  temperature,  but  must  all  have  been 
used  in  effecting  the  change  of  state  from  solid  ice  to 
liquid  water. 

In  the  same  manner  all  the  heat  which  was  applied  to 
the  water  while  it  was  changing  into  steam,  was  con- 
sumed in  changing  liquid  water  into  gaseous  steam. 
None  of  it  was  utilized  in  effecting  a  rise  in  temperature. 

105.  Quantity  of  Heat  Necessary  to  Change  Ice  to  Water. 
— Heat  is  consumed  in  changing  ice  to  water.  In  order 
to  determine  the  quantity  of  heat  necessary  for  this  pur- 
pose, the  experiment  must  be  conducted  so  that  all  the 
heat  used  shall  be  employed  solely  for  converting  ice 
into  water.  None  of  it  must  be  used  to  raise  the  temper- 
ature of  ice  from  some  lower  temperature  up  to  the  melt- 
ing point,  32°  F.,  and  none  of  it  must  be  used  to  raise 
the  water  resulting  from  the  melting  of  the  ice  from  its 
original  temperature,  32°  F.,  to  any  higher  temperature. 
This  can  be  accomplished  if  the  ice  experimented  upon 
has  a  temperature  of  32°  F.  at  the  beginning  of  the  ex- 
periment, and  if  the  experiment  is  stopped  the  instant  all 
the  ice  has  changed  to  water,  before  any  of  the  vrLi/er 
formed  by  the  ice  has  been  raised  above  its  original 
temperature. 

From  a  number  of  experiments  it  has  been  learned 
that,  if  one  pound  of  ice  having  a  temperature  of  32°  F. 
be  placed  in  one  pound  of  water  at  174°  F.,  all  of  the  ice 
will  be  melted  by  means  of  the  heat  derived  from  the 


HEAT  115 

pound  of  hot  water.  As  fast  as  tne  ice  melts,  the  water 
thus  formed  mingles  with  the  water  used  in  the  melting 
of  the  ice.  The  temperature  of  this  mixture  falls  as  the 
ice  melts,  and  after  the  ice  is  all  melted  the  temperature 
of  the  water  (including  both  the  original  water  used  and 
that  derived  from  the  melted  ice),  is  found  to  be  32°  F. 
At  this  moment,  the  temperature  of  both  the  water  re- 
sulting from  the  melting  ice  and  the  water  whose  orig- 
inal temperature  was  174°  is  found  to  be  the  same, 
namely,  32°.  This  was  also  the  temperature  of  the  ice. 
The  ice  turned  to  water  without  any  change  of  temper- 
ature. The  temperature  of  the  pound  of  water  used  in 
melting  the  ice  fell  142°. 

The  change  of  state  from  ice  to  water  was  caused  by 
the  quantity  of  heat  supplied  by  the  hot  water  during  its 
fall  of  temperature.  Since,  to  fall  one  degree,  a  pound 
of  water  must  give  out  1  calorie,  to  fall  142  degrees  it 
must  give  out  142  calories.  Therefore  142  calories  were 
consumed  in  melting  the  pound  of  ice.  One  hundred 
and  forty-two  calories  of  heat  may  also  be  supplied  by 
2  pounds  of  water  falling  71°,  or  by  3  pounds  of  water  fall- 
ing 47.33  degrees.  If,  therefore,  a  pound  of  ice  at  32°  is 
placed  in  two  pounds  of  water  at  a  temperature  of  103° 
F.,  or  in  three  pounds  of  water  at  79.33°  F.,  the  same 
result  is  obtained  as  in  the  preceding  experiment.  Suffi- 
cient heat  is  given  out  to  melt  all  of  the  ice.  If  less  heat 
is  supplied,  all  of  the  ice  does  not  melt.  If  more  heat  is 
supplied,  the  ice  firsts  melts  and  then  the  water  thus 
formed  is  raised  in  temperature. 

106.  Total  Quantity  of  Heat  Necessary  to  Melt  Ice  and  to 
Raise  the  Temperature  of  the  Resulting  Water. — If  it  be  de- 
sired to  change  a  pound  of  ice  at  32°  to  water  at  a  tem- 
perature of  say  60°,  this  can  be  effected  by  placing  the 


116  ELEMENTAEY  PHYSICS 

ice  in  water  of  such  quantity  and  temperature  that  it  will 
give  up  sufficient  heat  both  to  change  the  ice  to  water 
and  also  to  raise  the  water  from  a  temperature  of  32°  to 
that  of  60°.  The  amount  necessary  is  a  total  of  170  cal- 
ories. This  can  be  secured  by  taking  2  pounds  of  water 
having  a  temperature  of  145°,  or  five  pounds  of  water 
having  a  temperature  of  94°. 

One  hundred  and  eighty  calories  of  heat  are  necessary 
to  raise  a  pound  of  water  from  a  temperature  of  32°,  the 
freezing  point,  to  a  temperature  of  212°,  the  boiling  point. 
Almost  as  many  calories  of  heat,  142,  are  necessary  merely 
to  change  the  ice  to  water,  without  raising  its  temperature 
at  all.  All  of  this  heat  is  necessary  to  weaken  the  cohe- 
sion between  the  molecules  to  such  an  extent  that  the 
molecules,  which  are  only  moderately  movable  in  the 
case  of  ice,  may  change  their  position  with  the  greatest 
of  readiness  in  the  form  of  water.  This  ready  change  of 
position  of  the  molecules  is  characteristic  of  all  sub- 
stances while  in  the  liquid  state. 

Instead  of  using  the  heat  supplied  by  water,  the  heat 
given  out  by  a  Bunsen  burner  may  be  used  in  these  ex- 
periments. But  in  that  case  it  would  be  difficult  to  esti- 
mate the  number  of  calories  of  heat  supplied  by  the 
burner  during  the  change  of  ice  to  water. 

107.  Bough  Estimation  of  Quantity  of  Heat  Necessary  to 
Change  Water  to  Steam. — With  the  same  source  of  heat,  for 
instance  the  flame  from  a  small  Bunsen  burner,  it  takes 
about  5|  times  as  long  to  change  any  quantity  of  boiling 
water  into  steam  as  it  does  to  raise  the  same  quantity 
of  water  from  the  freezing  point,  32°,  to  the  boiling  point, 
212°.  Therefore  it  may  be  said  that  it  takes  about  5^ 
times  as  much  heat  to  convert  water  into  steam  as  it 
does  to  raise  water  from  the  freezing  to  the  boiling  point. 


HEAT  117 

Since  it  requires  180  calories  to  raise  one  pound  of  water 
from  the  freezing-  to  the  boiling  point,  it  will  require 
5J  x  180,  or  about  960  calories,  to  convert  one  pound  of 
water  at  212°  into  steam.  This  is  only  a  rough  estimate, 
and  if  the  experiment  be  attempted,  the  results  might 
not  be  so  satisfactory,  but  it  will  serve  to  give  a  concep- 
tion of  the  quantity  of  heat  necessary  to  cause  the  con- 
version of  water  into  steam. 

108.  The  Quantity  of  Heat  Necessary  to  Change  Water 
into  Steam  may  be  Determined  Indirectly. — The  amount  of 
heat  necessary  to  change  water  into  steam  is  not  usually 
determined  directly.     Instead  of  finding  how  many  cal- 
ories of  heat  are  necessary  to  change  a  pound  of  water 
into  a  pound  of  steam,  the  experimenter  determines  how 
many  calories  of  heat  the  steam  is  able  to  give  out  when 
it  returns  from  its  condition  of  steam  to  that  of  water.   If, 
for  instance,  a  certain  quantity  of  steam  at  a  temperature 
of  212°,  on  changing  to  water  at  a  temperature  of  212°, 
can  be  shown  to  give  out  100  calories  of  heat,  it  may 
justly  be  assumed  that  100  calories  of  heat  are  necessary 
also  to  change  that  weight  of  water  to  steam. 

The  number  of  calories  given  off  by  the  steam  may  be 
determined  by  noticing  the  rise  in  temperature  produced 
in  one  pound  of  water  by  a  given  quantity  of  steam.  If, 
for  example,  any  given  quantity  of  steam  is  found  to 
raise  the  temperature  of  one  pound  of  water  from  60°  to 
80°  it  is  a  safe  conclusion  that  the  steam  has  given  out  20 
calories  of  heat.  This  is  the  method  used  in  the  follow- 
ing experiment. 

109.  Exact  Determination  of  Quantity  of  Heat  Necessary 
to  Change  Water  into  Steam. — Place  a  Florence  flask  on  a 
piece  of  wire-gauze  covering  one  of  the  rings  of  an  iron 
ring  stand   (Fig.   64).      Half  fill   the  flask  with  water.. 


118 


ELEMENTARY   PHYSICS 


Close  the  flask  with  a  one -hole  rubber  stopper  and 
through  the  single  opening  passing  through  the  stop- 
per thrust  one  end  of  a  glass  tube. 

At  some  distance  from  the  Florence  flask  place  a  sec- 
ond vessel.  Pour  into  the  vessel  a  known  quantity  of 
water,  for  instance,  one  pound.  Close  the  vessel  with  a 
rubber  stopper  perforated  by  means  of  three  openings. 
Through  one  opening  thrust  a  glass  tube  so  that  its  lower 
end  will  extend  nearly  to  the  bottom  of  the  water  in  the 


FIG.  64. 

vessel,  and  connect  the  upper  end  of  this  tube  with  the 
Florence  flask.  Through  the  second  opening  insert  a 
chemical  thermometer  so  that  the  bulb  will  dip  into  the 
water.  Through  the  third  opening  thjust  a  glass  tube 
bent  in  such  a  manner  that  if  any  of  the  escaping  steam 
condenses  into  water,  the  water  will  not  be  able  to  return 
to  the  vessel.  Determine  the  temperature  of  the  water 
in  the  vessel  carefully  by  means  of  the  thermometer. 
Disconnect  the  Florence  flask  temporarily  from  the  ves- 


HEAT  119 

sel  and  heat  the  water  in  the  flask  until  steam  is  given 
off  freely.  Then  quickly  reconnect  the  flask  with  the 
vessel  and  continue  the  formation  of  steam  in  the  flask. 

The  steam  formed  in  the  flask  will  leave  by  means  of 
the  glass  tube.  It  will  pass  along  the  tube  and  will  es- 
cape at  its  other  end  into  the  water  which  occupies  the 
lower  part  of  the  vessel.  As  the  steam  rises  through 
this  water,  it  is  cooled  by  the  water  and  returns  to  the 
liquid  form.  In  so  doing  it  gives  up  to  the  water  that 
part  of  the  heat  which  was  formerly  necessary  to  convert 
the  steam,  from  its  original  liquid,  to  its  later  gaseous 
form.  Any  steam  which  escapes  from  the  vessel  must 
still  have  a  temperature  of  212°,  otherwise  it  would  no 
longer  be  in  the  form  of  steam,  and,  since  it  cannot  have 
given  up  any  part  of  its  heat  to  the  water,  its  escape  will 
not  in  any  way  affect  the  results  of  the  experiment. 

If  the  pound  of  water  at  the  beginning  of  the  experi- 
ment had  a  temperature  of  60°  F.,  and,  if  after  the  en- 
trance of  steam  for  some  time,  its  temperature  has  been 
raised  to  212°,  the  steam  has  supplied  152  calories  of  heat 
to  the  water  in  the  vessel.  It  is  now  necessary  to  deter- 
mine how  much  steam  was  necessary  to  supply  152  cal- 
ories of  heat.  This  may  be  determined  by  weighing  the 
water  as  soon  as  its  temperature  has  arrived  at  212°  and 
subtracting  from  this  the  weight  of  the  water  at  the  be- 
ginning of  the  experiment.  The  additional  weight  is 
evidently  due  to  the  addition  of  that  part  of  the  steam 
which  was  condensed  on  entering  the  vessel. 

If  the  weight  of  the  water  added  to  the  vessel  be  .156  H- 
of  a  pound,  it  is  evident  that  .156+  of  a  pound  of  steam 
was  able  to  raise  the  temperature  of  1  pound  of  water  from 
60°  F.  to  212°  F.  Therefore,  .156  +  of  a  pound  of  steam 
gives  up  152  calories  of  heat  on  changing  from  steam  at 


120  ELEMENTARY  PHYSICS 

212°  to  water  at  212°.  It  must,  consequently,  have  re- 
quired 152  calories  of  heat  to  convert  .156  +  of  a  pound  of 
water,  at  a  temperature  of  212°,  to  steam  at  212°. 

From  this  it  may  be  calculated  that  it  would  have  re- 
quired about  965  calories  of  heat  to  convert  an  entire 
pound  of  water,  at  212°,  to  steam  at  212°. 

In  raising  one  pound  of  water  from  ice  at  a  tempera- 
ture of  0°  to  steam  at  a  temperature  of  250°,  1321.3  cal- 
ories are  needed.  Of  these  only  214.3  calories  are  used 
in  raising  the  temperature.  The  remainder  are  utilized 
in  changing  the  ice  to  water  and  the  water  to  steam. 
In  the  diagram  (Fig.  65)  the  number  of  calories  needed 
to  raise  the  temperature  is  indicated  by  the  length  of  the 

Steam  250° 
Water  212^ ^ 18.2  -^ 


Steam  212-° 

16.1  +  142  +180  +965  +18.2  -1321.3  Calories 
o£  «hich  only  214.3  raise  the  temperature.. 


^  Water  32° 

FIG.  65. 

vertical  lines  and  the  number  needed  to  change  ice  to 
water,  and  water  to  steam,  by  the  length  of  the  horizon- 
tal lines. 

110.  Latent  Heat  of  Water  and  Steam. — When  the  changes 
of  ice  to  water,  and  of  water  to  steam  were  first  studied, 
observers  merely  noticed  that  the  mercury  in  the  ther- 
mometers failed  to  rise  during  the  time  in  which  these 
changes  were  taking  place,  while  at  other  times  any  addi- 
tion of  heat  was  at  once  followed  by  a  rise  in  tempera- 
ture. Observers,  therefore,  said  that  during  the  change 
of  a  substance  from  a  solid  to  a  liquid  state,  and  later 
again  during  the  change  from  a  liquid  to  a  gaseous  state, 
a  definite  quantity  of  heat  in  some  way  became  hidden. 


HEAT  121 

The  heat  which  disappeared  while  these  changes  were 
taking  place  was  called  latent  heat,  the  word  "  latent " 
meaning  to  lie  hidden  or  concealed.  Such  expressions 
as  the  latent  heat  of  water  and  the  latent  heat  of  steam 
are  still  in  common  use.  In  the  preceding  paragraphs 
it  has  been  shown  that  the  latent  heat  of  water  is  equal 
to  142  calories,  and  the  latent  heat  of  steam  to  965  cal- 
ories. 

When  steam  returns  to  water,  or  water  to  ice,  the  latent 
heat  is  again  given  out. 

111.  Importance  of  the  Large  Latent  Heat  of  Water. — Ev- 
ery pound  of  water,  on  changing  from  water  at  a  tempera- 
ture of  32°  to  ice  at  32°  F.,  must  give  out  142  calories  of 
heat.  This  quantity  of  heat  must  escape  before  the  next 
pound  of  water  can  turn  into  ice. 

When  the  water  at  the  surface  of  a  lake  or  river  freezes, 
it  not  only  gives  out  heat  to  the  air  above  but  also  to  the 
water  below.  This  causes  the  freezing  of  lakes  and  rivers 
to  be  very  slow.  Therefore,  the  depth  to  which  water  is 
frozen  is  very  slight  compared  to  the  depth  to  which  it 
would  be  frozen,  if  the  heat  which  it  gave  out  on  chang- 
ing from  a  liquid  to  a  solid  state  were  very  small. 

During  very  cold  weather,  when  the  vegetables  in  the 
cellar  are  in  danger  of  freezing,  some  people,  in  order 
to  prevent  serious  injury,  place  a  tub  of  water  near  the 
vegetables.  Every  pound  of  water  which  freezes  gives 
out  142  calories  of  heat.  This  heat  becomes  dispersed 
throughout  the  cellar,  and  the  total  quantity  of  heat 
given  out  is  often  sufficient  to  prevent  serious  injury  to 
the  vegetables.  Fortunately,  most  winter  vegetables  do 
not  suffer  from  cold  until  a  temperature  of  several  de- 
grees below  the  freezing  point  has  been  reached. 

When  ice  is  melted  in  the  laboratory,  the  heat  necessary 


122  ELEMENTARY   PHYSICS 

to  change  ice  into  water  is  supplied  by  a  Bunsen  burner 
or  some  other  artificial  source  of  heat.  In  this  case  the 
source  of  heat  is  evident.  However  when  ice  melts  on  ex- 
posure to  the  open  air,  without  any  especial  attempt  being- 
made  to  supply  heat  to  the  ice,  the  fact  that  it  requires  a 
considerable  quantity  of  heat  to  melt  every  pound  of  ice 
is  likely  to  be  overlooked.  From  what  source  is  this 
heat  obtained?  From  the  surrounding  objects. 

The  fact  that  ice,  in  order  to  melt,  withdraws  heat  from 
surrounding  objects  may  be  shown  in  a  number  of  ways. 
The  ice  in  an  ice-chest,  on  melting,  withdraws  heat  from 
the  objects  placed  in  the  chest.  When  snow  melts  in  the 
early  spring,  the  air  frequently  feels  very  chilly.  So 
much  heat  is  withdrawn  from  surrounding  objects,  includ- 
ing both  the  air  and  the  snow  in  the  immediate  neighbor- 
hood, as  to  prevent  the  rapid  melting  of  the  snow.  If  it 
were  not  for  this  fact,  disastrous  floods,  similar  to  those 
which  are  often  caused  at  present  by  thaws  which  follow 
a  considerable  fall  of  snow,  would  be  far  more  extensive 
and  frequent.  Near  Lake  Erie,  spring  is  often  so  much 
retarded  by  the  melting  of  the  winter's  ice  and  snow,  that 
the  buds  of  trees  usually  do  not  begin  growth  until  the 
danger  of  late  frost  is  past. 

112,  Importance  of  the  Large  Latent  Heat  of  Steam. — The 
fact,  that  it  requires  965  calories  of  heat  to  convert  one 
pound  of  water  into  steam,  makes  necessary  the  consump- 
tion of  enormous  quantities  of  coal  for  the  production 
of  steam  in  boilers.  The  heat  which  is  given  out,  when 
steam  returns  to  the  liquid  condition,  is  utilized  in  the 
heating  of  buildings.  Every  pound  of  steam  which  con- 
denses to  water  inside  of  a  radiator,  used  for  heating, 
supplies  965  calories  of  heat  to  the  room.  As  the  con- 
densed water  returns  to  the  boiler,  it  continues  to  give 


HEAT  123 

out  heat,  in  consequence  of  the  water  falling  to  a  tem- 
perature below  212°. 

113.  Heat  Withdrawn  by  the  Evaporation  of  Liquids. — 
When  water  is  converted  into  vapor  by  means  of  ordinary 
evaporation,  heat  is  consumed  in  the  same  manner  as 
when  water  is  converted  into  steam,  but  the  process  is 
much  slower.     The  heat  which  is  used  to  convert  the 
water  to  vapor  is  withdrawn  from  surrounding  objects. 
After  a  shower  of  rain  the  water  evaporates  and  cools  the 
air  by  withdrawing  from  the  air  a  portion  of  its  heat. 
The  evaporation  of  water,  sprinkled  on  the  floor,  cools 
the  air  of  a  warm  room.     The  evaporation  of  perspira- 
tion during  a  hot  day  serves  to  cool  the  body.     When 
the  air  is  already  saturated  with  moisture,  so  that  the 
perspiration  fails  to  evaporate  promptly,  much  discom- 
fort is  felt.    Fanning  causes  a  more  rapid  evaporation  of 
the  perspiration,  and  thereby  produces  a  refreshing  cool- 
ness.    In  tropical   countries  water  is   cooled  by  being 
placed  in  porous  jars  set  in  a  draught  of  air.     A  small 
quantity  of  water  passes  through  the   pores  of  the  jar, 
and,  on  evaporating,  withdraws  so  much  heat  from  the  jar 
that  the  temperature  of  the  water  remaining  in  the  jar  is 
lowered  considerably  below  that  of  water  contained  in 
any  vessel  which  does  not  permit  ready  evaporation. 

Ether,  when  thrown  in  a  fine  spray,  evaporates  so 
rapidly  and  withdraws  such  great  quantities  of  heat 
from  surrounding  objects  that,  when  thrown  on  any  part 
of  the  body  this  part  becomes  numb  on  account  of  the 
cold.  In  this  manner,  parts  of  the  body  are  rendered 
insensible  to  pain  during  surgical  operations. 

114,  Cold  Produced  by  Solution — When  a  substance  dis- 
solves, it  turns  from  a  solid  into  what  may  be  considered 
a  liquid  form.     It  requires  heat  to  enable  it  to  make  this 


124  ELEMENTARY  PHYSICS 

change  of  state.  The  necessary  heat  is  withdrawn  from 
surrounding  objects,  the  chief  of  which  is  the  liquid  in 
which  the  substance  dissolves. 

If  a  certain  quantity  of  the  solid,  called  ammonium 
nitrate,  is  thrown  into  an  equal  weight  of  water,  the  solid 
dissolves  and  the  temperature  of  the  water  falls  about 
45°  F.  If  four  parts,  by  weight,  of  crystallized  chloride 
of  calcium  are  mingled  with  three  parts  of  snow,  both 
the  chloride  of  calcium  and  the  snow  become  liquefied 
and  the  temperature  of  the  mixture  falls  about'  86°.  If 
one  part,  by  weight,  of  salt  be  mixed  with  two  parts  of 
finely  broken  ice,  both  are  liquefied,  and  if  care  is  taken 
to  exclude  the  surrounding  heat,  the  temperature  is 
lowered  nearly  38°.  The  salt  causes  the  ice  to  melt  more 
rapidly  than  the  ice  would  melt  if  the  salt  were  not  pres- 
ent. The  result  is  that  a  much  greater  degree  of  cold  is 
produced  than  would  be  the  case  if  the  ice  were  melted 
alone.  For  this  reason  the  cream  in  an  ice-cream  freezer 
is  surrounded  by  a  mixture  of  ice  and  salt.  Even  if  salt 
alone  were  placed  in  water  the  temperature  of  the  water 
would  be  lowered  to  a  perceptible  extent  (§  193). 

115.  Gases  Cool  as  they  Expand. — A  gas  can  be  caused 
to  expand  by  increasing  the  quantity  of  heat  which  it 
contains.  This  may  be  accomplished  by  means  of  a  Bun- 
sen  burner  or  some  other  source  of  heat.  It  requires 
heat  to  cause  molecules  to  move  apart.  If  a  gas  is  al- 
lowed to  expand,  by  reducing  the  pressure  which  is 
exerted  upon  it,  the  heat  must  be  secured  from  some 
other  source.  This  source  of  heat  is  the  gas  itself  to- 
gether with  all  the  objects  enclosed  within  and  surround- 
ing the  gas.  Therefore,  when  a  gas  expands  the  tempera- 
ture of  the  gas  is  found  to  decrease. 

This  principle  is  illustrated  in  the  formation  of  clouds. 


HEAT  125 

Air  can  absorb  a  certain  quantity  of  moisture.  This 
quantity  increases  with  the  temperature  of  the  air. 
When  air  is  heated  it  rises  (§  127).  As  it  ascends  to 
higher  elevations,  the  heated  air  is  subjected  to  less 
pressure  from  the  atmosphere.  It  therefore  expands, 
and,  in  order  to  expand,  heat  is  necessary.  This  heat  is 
secured  from  the  air  and  the  moisture  which  it  contains. 
In  consequence,  the  temperature  of  the  air  falls.  When 
air  cools,  it  can  no  longer  hold  as  much  moisture.  A  part 
of  the  moisture,  therefore,  leaves  the  air  and  collects  in 
minute  drops.  If  the  number  of  these  drops  is  large, 
they  may  be  seen  some  distance  above  the  earth  in  the 
form  of  a  cloud.  The  drops  of  water  are  so  small  and  so 
light  that  they  seem  to  remain  suspended  in  the  air  at 
nearly  the  same  levels.  In  reality,  however,  they  settle 
slowly  through  the  air,  but  on  reaching  the  warmer  air 
below  are  reabsorbed  again,  so  that  the  falling  of  the 
little  drops  of  moisture  which  form  clouds  does  not  attract 
attention. 

116.  Capacity  of  Different   Substances  for   Heat. — Some 
substances  ha,ve  a  greater  capacity  for  heat  than  others. 
For  instance,  the  same  amount  of  heat  which  will  raise 
one  pound  of  water  one  degree  would  be  sufficient  to 
raise  the  temperature  of  33  pounds  of  lead  one  degree. 
In  other  words,  water  has  33  times  more  capacity  for 
heat  than  lead.     Copper  has  about  three  times  as  much 
capacity  for  heat  as  lead.     Iron  has  nearly  four  times  as 
much.     On  account  of  the  great  capacity  of  water  for 
heat,   water  is  taken   as  the   standard  with   which  the 
capacity  of  other  substances  is  compared. 

117.  The  Irregular  Expansion  of  Water, — As  a  rule,  any 
increase  in  the  quantity  of  heat  contained  in  any  sub- 
stance causes  it  to  expand.     This  rule  applies  not  only  to 


126  ELEMENTARY   PHYSICS 

substances  while  in  the  solid,  liquid,  or  gaseous  condition, 
but  also  to  substances  while  changing  from  a  solid  to  a 
liquid,  or  from  a  liquid  to  a  gaseous  state. 

Water  is  one  of  the  most  striking  exceptions  to  this 
rule.  When  water  changes  from  the  solid  to  the  liquid 
state,  it  contracts.  In  the  liquid  state  it  occupies  only 
.917  of  the  volume  occupied  by  the  ice  just  before  melt- 
ing. The  result  is  that  ice  floats  in  water. 

Most  liquids  expand  on  being  heated  from  the  melting 
to  the  boiling  point.  They  do  not  expand  equally  for 
every  equal  increase  of  temperature,  but  they  at  least 
expand. 

Water,  on  the  contrary,  at  first  contracts  and  then  ex- 
pands. From  32°  it  contracts  until  a  temperature  of  39.2° 
is  reached;  from  39.2°  to  212°  it  expands.  The  rate  of 
contraction  and  of  expansion  is  unequal.  From  32°  to 
39.2°  the  rate  of  contraction  decreases  until  at  39.2°  F.  the 
rate  of  contraction  is  very  slight.  Above  39.2°  the  rate  of 
expansion  is  at  first  very  slight  and  then  increases  until 
the  boiling  point  is  reached.  At  200°  F.,  for  instance,  an 
increase  in  temperature  of  1°  F.  causes  an  increase  in  vol- 
ume six  times  as  great  as  a  corresponding  increase  of 
temperature  at  50°. 

The  total  decrease  of  volume  between  32°  and  39.2°  F. 
is  onlyjpoQ-of  the  original  volume.  The  total  increase  of 
volume  on  raising  water  from  a  temperature  of  39.2°  to 
that  of  212°  F.,  is  about  |s  of  its  volume  at  39.2°  F.  Ice, 
therefore,  floats  in  water  at  any  temperature. 

118.  Heat  Produced  by  Collision. — Place  a  small  piece  of 
iron  on  an  anvil  and  pound  it  vigorously  with  a  hammer. 
It  soon  becomes  warm,  and,  if  it  be  pounded  for  a  suf- 
ficient length  of  time,  it  will  become  too  hot  to  be  held  in 
the  hand. 


HEAT  127 

Pounding1  causes  the  molecules  at  the  surface  of  the 
iron  to  move  violently.  The  molecules  at  the  surface 
strike  the  molecules  just  beneath  and  set  these  also  in 
motion.  Molecules  strike  against  molecules,  rebound,  and 
strike  against  other  molecules,  until,  finally,  all  the  mol- 
ecules within  the  body  take  part  in  the  motion.  In  the 
meantime,  if  the  pounding  be  kept  up,  the  heat  of  the 
piece  of  iron  continually  increases. 

119.  Heat  Produced  by  Compression  and  Friction. — When 
a  bullet  is  fired  against  a  stone,  the  lead  of 
which  the  bullet  is  composed  is  violently  com- 
pressed and  becomes  hot.  As  the  lead  is 
stopped  by  the  stone,  the  concussion  is  suffi- 
cient to  set  the  molecules  of  lead  into  violent 
motion.  Increase  of  heat  is  the  result. 

A  sudden  compression  of  air  will  also  produce 
heat.  Attach  a  small  piece  of  dry  tinder  to  the 
end  of  a  piston  fitting  air-tight  in  the  cylinder 
of  a  fire  syringe,  and  plunge  the  piston  quickly 
into  the  cylinder  (Fig.  66).  During  the  com- 
pression of  the  air  within  the  cylinder,  the  mole- 
cules of  air  are  set  in  motion  and  this  generates 
sufficient  heat  to  ignite  the  tinder.  FlG  66 

Hold  a  piece  of  iron  against  the  edge  of  a 
dry,  rapidly  revolving  grindstone.   The  grindstone  knocks 
off  tiny  fragments  of  the  iron.     These  fragments  are  so 
heated  as  a  result  of  the  concussion  that,  as  they  fly  off, 
they  produce  the  appearance  of  a  stream  of  sparks. 

Rub  the  knuckles  vigorously  back  and  forth  on  the 
sleeve  of  your  coat.  The  friction  of  the  knuckles  against 
the  sleeve  sets  the  molecules  in  violent  motion  and  the 
knuckles  become  hot.  Indians  start  fires  by  rapidly 
whirling  the  end  of  a  soft  piece  of  wood  in  the  hollow 


128  ELEMENTARY  PHYSICS 

formed  on  the  upper  surface  of  a  harder  piece.  The  fric- 
tion produces  enough  heat  to  set  the  softer  wood  on  fire. 

120.  Heat  a  Form  of  Molecular  Motion. — Collision,  com- 
pression, and  friction,  all  cause  molecular  motion,  and 
all  result  in  an  increase  of  heat.  This  suggests  that  heat 
is  the  result  of  molecular  motion,  and  that  increase  in 
molecular  motion  results  in  an  increase  of  heat.  When 
the  temperature  of  a  body  is  increased,  the  molecules  are 
set  in  more  violent  motion.  They  strike  each  other  with 
such  force  that  they  knock  each  other  farther  apart,  and, 
in  consequence,  the  body  expands.  When  a  body  cools, 
the  motion  of  its  molecules  becomes  less  violent,  the  mol- 
ecules do  not  knock  each  other  so  far  apart,  and  the  vol- 
ume of  the  body  decreases  (§§  61,  62,  and  63). 

The  nature  of  these  molecular  motions  is  not  fully  un- 
derstood. The  motions  so  far  described  partake  chiefly 
of  the  nature  of  a  bombardment.  The  molecules  move  in 
one  direction  until  they  come  into  violent  contact  with 
other  molecules,  when  they  rebound  and  move  in  some 
other  direction.  This  form  of  motion  from  place  to  place 
may  be  called  a  motion  of  translation. 

When  one  molecule  strikes  against  another  molecule, 
there  is  also  another  form  of  motion  in  addition  to  the 
motion  of  translation.  The  molecule  struck  apparently  is 
caused  to  tremble.  Since  we  do  not  exactly  know  what 
a  molecule  is,  or  what  form  it  possesses,  we  do  not  have 
a  very  exact  idea  as  to  the  nature  of  the  trembling  which 
takes  place  in  the  material  of  a  molecule  after  it  has  been 
struck.  But  we  may  get  some  conception  of  such  a  mo- 
tion by  studying  the  motions  of  a  very  elastic  rubber  ball. 

When  a  rubber  ball  (Fig.  67,  a)  is  struck  violently  it 
flattens  for  a  moment  transversely  to  the  direction  of 
the  blow  (b).  Then  the  displaced  particles  return  so 


HEAT 


129 


violently  to  their  original  positions  that  they  do  not 
cease  their  motion  when  the  ball  has  resumed  its  original 
shape,  but  for  a  moment  the  particles,  which  were  moving- 
outward,  continue  to  move  outward,  and  those  which 
were  moving-  inward  continue  to  move  inward.  The  re- 
sult is  that  the  flattened  ball  first  returns  to  its  origi- 
nal shape  (c)  and  then  assumes  the  form  of  a  slightly 
elongated  football  (d).  Then  the  particles  which  have 
been  moving  outward,  return,  and  those  which  have 
been  moving  inward,  move  outward ;  until  the  flattened 


FIG.  67. 

condition,  assumed  at  first  (b),  is  re- 
sumed. This  action  continues  for 
some  time,  becoming  less  violent  if 
no  further  concussions  occur,  until 
the  motion  finally  ceases.  This  is 
one  of  the  many  possible  forms  of  vibration.  The  con- 
cussion of  molecules  against  each  other  may  give  to  them 
a  vibratory  motion  somewhat  of  this  character. 

The  motions  which  cause  the  molecular  phenomena 
called  heat  are,  probably,  a  combination  of  the  motions 
of  vibration  and  translation,  and  possibly  also  of  other 
forms  of  motion,  such  as  rotation. 

121.  Conduction. — Place  one  end  of  a  very  short  bar  of 
iron  on  an  anvil  and  pound  this  end  vigorously  with  a 
hammer.  Not  only  will  the  end  which  was  pounded 


130  ELEMENTARY  PHYSICS 

become  hot,  but,  if  the  pounding-  is  continued  long- 
enough,  the  opposite  end,  which  was  not  struck,  will  also 
become  distinctly  warmer.  The  violent  motions  of  the 
molecules  struck  by  the  hammer  are  communicated  from 
molecule  to  molecule  until  even  the  molecules  at  the  ex- 
treme end  of  the  bar  of  iron  are  set  in  violent  motion. 

Hold  in  your  hand  one  end  of  an  iron  rod  about  eight 
inches  long,  and  place  the  other  end  in  the  flame  of  a 
Bunsen  burner.  The  end  held  in  the  hand  also  becomes 
hot.  The  molecules  brought  into  contact  with  the  flame 
are  caused  to  move  violently  on  account  of  the  heat,  and 
this  motion  is  communicated  from  molecule  to  molecule 
until  the  molecules  at  the  end  held  in  the  hand  also  move 
violently. 

In  the  case  just  described,  both  the  bar  and  the  rod 
maintain  their  general  form,  showing  that  while  these 
molecules  are  in  motion  they  do  not  change  their  relative 
positions.  If  there  were  any  considerable  change  in  the 
relative  positions  of  the  molecules,  the  general  form  of 
the  bar  and  rod  could  not  be  maintained.  The  communi- 
cation of  heat  from  one  part  of  a  body  to  another  caused 
by  communication  of  motion  from  molecule  to  molecule, 
not  accompanied  by  any  change  in  the  relative  positions 
of  the  molecules,  is  called  conduction. 

122.  Relative  Conductivity  of  Different  Solids. — As  a  rule 
the  metals  are  the  best  conductors  of  heat.  But  different 
metals  vary  greatly  in  the  rapidity  with  which  they  con- 
duct heat.  Copper  conducts  heat  about  four  times  as 
effectively  as  iron.  Rocks  are  much  poorer  conductors 
of  heat.  The  conductivity  of  sandstone,  for  instance,  is 
only  one -twentieth  of  that  of  iron.  The  various  kinds  of 
wood  are  still  poorer  conductors.  The  wood  of  the  fir-tree, 
for  example,  conducts  heat  only  ffa  as  readily  as  iron. 


HEAT  131 

The  temperature  of  the  human  body  is  98.5°  F.  If  on 
a  cold  winter  morning-,  when  the  temperature  of  the  air 
in  a  room  and  of  all  the  objects  within  the  room  is  about 
32°  F.,  a  person  should  step  with  bare  feet  in  succession 
on  a  piece  of  iron,  a  piece  of  wood,  and  the  carpet,  the 
iron  would  feel  colder  than  the  wood,  and  the  wood  would 
feel  colder  than  the  carpet.  Iron  conducts  the  heat  away 
from  the  foot  more  rapidly  than  wood,  and  wood  conducts 
the  heat  away  more  rapidly  than  the  woollen  carpet.  In 
consequence,  the  temperature  of  the  foot  is  lowered  much 
more  rapidly  when  in  contact  with  the  iron  than  when  in 
contact  with  the  carpet.  Therefore  the  iron  feels  colder, 
although  its  temperature  is  practically  the  same  as  that 
of  the  carpet. 

Clothing  for  summer  use  should  be  made  of  material 
which  conducts  heat  away  from  the  body  fairly  well. 
Linen  and  cotton  are  the  best  materials  in  common  use. 
During  the  winter,  clothing  should  be  made  of  the  poor- 
est conductors  of  heat.  Wool  is  one  of  the  best  non- 
conductors in  common  use.  Furs  are  used  extensively  in 
northern  countries.  The  feathers  and  down  of  northern 
birds  are  such  effective  non-conductors  that  in  some 
countries  they  are  sewn  up  in  ticks  and  in  this  form  are 
used  in  the  place  of  quilts  as  a  covering  for  beds  during 
the  winter  months.  The  non-conductivity  of  loose  wool, 
furs,  and  feathers  is  largely  due  to  the  air  imprisoned 
between  the  fibres  or  hairs  forming  the  material. 

Paper  is  a  good  non-conductor.  In  early  spring,  and 
late  fall,  plants  may  be  protected  by  covering  them  with 
paper.  If,  on  a  very  cold  night,  several  layers  of  ordi- 
nary newspaper  be  placed  between  the  quilts  on  a  bed, 
the  warmth  of  the  body  will  be  better  kept  up.  Several 
newspapers  wrapped  around  the  body,  underneath  a 


132  ELEMENTARY   PHYSICS 

coat,  will  serve  as  a  very  effective  protection  against  the 
cold  in  stormy  winter  weather. 

123.  Heating  of  Glass  Vessels. — All  varieties  of  glass  are 
poor  conductors  of  heat.  If  hot  water  is  poured  into  a 
thick  glass  vessel  having  the  ordinary  temperature  of  the 
room,  the  interior  begins  to  expand  strongly  before  the 
heat  has  been  conducted  to  the  outer  surface,  and,  in  con- 
sequence, the  vessel  often  cracks.  For  similar  reasons  a 
thick  glass  vessel  is  likely  to  crack  if  heat  is  suddenly 
applied  to  the  exterior  by  means  of  a  Bunsen  burner. 
In  order  to  avoid  cracking  on  account  of  unequal  expan- 
sion, glass  vessels  used  for  chemical  experiments  requir- 
ing heat  are  usually  made  of  thin  glass. 

Even  thin  glass  vessels  require  the  use  of  certain  pre- 
cautions to  avoid  cracking.  The  greatest  heat  should 
not  be  applied  to  the  end  of  the  test-tube,  since  this  part 
of  the  tube  is  often  thinner  than  the  sides.  Apply  the 
heat  gradually,  moving  the  test-tube  in  and  out  of  the 
flame,  and  rolling  it  slightly  between  thumb  and  finger. 
Beakers  and  flasks  may  easily  become  unequally  heated 
and  crack.  In  order  to  avoid  this,  place  the  beaker  on  a 
piece  of  brass  wire-gauze  (Figs.  64,  71)  supported  by  a 
ring  stand.  Brass  conducts  heat  150  times  better  than 
glass.  Hence  the  brass  wire-gauze  will  conduct  the  heat 
so  rapidly  to  all  parts  of  the  base  of  the  beaker  or  flask, 
that  the  base  of  the  beaker  is  more  equally  heated  and 
the  risk  of  breaking  it  is  very  much  lessened.  But  even 
in  this  case  the  heat  should  be  applied  gradually  at  first. 

Do  not  apply  heat  to  that  part  of  a  test-tube  which  is 
on  a  level  with  the  top  of  the  liquid  inside.  The  dry 
glass  is  likely  to  become  so  much  warmer  than  that  which 
is  kept  cooler  by  contact  with  the  liquid  that  the  tube 
may  crack.  For  similar  reasons  do  not  place  in  the  di- 


HEAT 


133 


~F.IG.  68. 


rect  flame  any  test-tube  to  whose  walls  the  drops  of  any 
liquid  adhere. 

124.  Liquids  Poor  Conductors  of  Heat. — The  fact  that 
liquids  are  not  good  conductors  of  heat  can  be  shown  in 
quite  a  number  of  ways.  In  north- 
ern countries  large  fires  are  often 
built  on  the  ice  of  rivers  and  ponds, 
to  furnish  light  and  heat  to  skaters. 
As  soon  as  a  thin  layer  of  water  is 
formed  beneath  the  burning  wood, 
the  ice  almost  ceases  to  melt.  If  a 
metal  pan  is  floated  upon  water 
(Fig.  68),  and  alcohol  is  placed 
in  the  pan  and  set  on  fire,  the 
metal  pan  communicates  the  heat 
quickly  to  the  water,  but  the  lower 
part  of  the  water  remains  cold  for  a  very  long  time. 
If  the  flame  of  a  Bunsen  burner  is  directed  against  the 

upper  part  of  a  test  tube 
nearly  full  of  water  (Fig.  69), 
the  upper  part  of  the  water 
may  be  made  to  boil  while 
the  water  in  the  lower  part  of 
the  tube  remains  quite  cool. 
125.  Gases  are  Poor  Conductors. — Gases 
are  poor  conductors  of  heat.  It  has 
already  been  mentioned  that  the  air  en- 
closed in  loose  wool,  furs,  and  feathers, 
contributes  much  toward  the  non-con- 
ductivity of  these  substances.  The  air  enclosed  between 
double  doors  and  windows  also  serves  as  a  non-conductor, 
and  prevents  the  escape  of  heat  from  houses.  The  air 
enclosed  between  particles  of  sawdust  placed  within  the 


FIG. 


134 


ELEMENTARY   PHYSICS 


FIG,  70. 


walls  of  ice-houses  is  an  important  factor,  during-  the 
summer,  in  preventing*  the  heat  outside  of  the  ice-house 
from  gaining-  entrance  to  the  ice  within  (§§  217-223). 

126.  Convection  of  Liquids. — Notwithstanding  the  fact 
that  liquids  are  poor  conductors,  heat  is  often  readily 
communicated  from  one  part  of  the  liquid 
in  a  vessel  to  another  part  in  the  same 
vessel.  If  a  Bunsen  burner  be  placed  be- 
neath a  test-tube,  inclined  at  a  moderate 
angle,  the  water  at  the  base  of  the  tube 
becomes  hot  (Fig.  70).  It  expands,  be- 
comes lighter,  and,  in  consequence,  it  rises 
along  the  upper  side  of  the  inclined  tube 
and  floats  on  top  of 
the  water  not  yet 
heated.  At  the  same 
time  the  water,  which 
is  at  a  greater  distance  above  the 
flame,  and  which  is  still  cold,  comes 
down  along  the  lower  side  of  the 
inclined  tube,  and  takes  the  place 
of  the  water  which  is  rising.  This 
colder  water  becomes  heated  in  its 
turn  and  also  rises.  The  result 
is  the  formation  of  a  continu- 
ous current  which  can  be  easily 
recognized  if  fine  cork-dust  is 
thrown  into  the  water.  In  the 
same  manner,  irregular  currents  are  started  in  the  water 
in  tea-kettles,  or  in  coffee-pots,  when  placed  over  a  fire 
(Fig.  71). 

In  all  of  these  cases  the  heated  water  expands  and  be- 
comes lighter.     The  colder  water  is  heavier.     Both  kinds 


FIG.  71. 


HEAT 


135 


of  water  are  pulled  down  by  the  attractive  force  of  the 
earth  (gravitation).  Since  the  heavier  water  is  pulled 
down  with  greater  force,  the  heavier  water  will  be  drawn 
beneath  the  lighter  water,  and  the  lighter  water  in  con- 
sequence will  then  rise  to  the  top. 

The  water  at  the  surface  of  the  ocean  within  the  tropics 
becomes  much  more  heated  than  that  at  the  surface  within 
the  Arctic  regions.  In  consequence,  the  colder  water  of 
the  Arctic  ocean  gradually  and  very  slowly  settles  be- 
neath the  warmer  water  of  more  southern  regions,  while 
the  heated  water  of  the  Torrid  zone  gradually  passes 
northward  along  the  surface  un- 
til it  reaches  the  Arctic  Ocean. 
This  is  one  of  the  causes  which 
produce  currents  in  the  ocean. 

127.  Convection  of  Gases.  — 
When  air  is  unequally  heated, 
so  that  one  part  of  the  air  be- 
comes hot  while  the  other  re- 
mains cold,  the  hot  air  becomes 
lighter  than  the  cold  air.  The 
cold  air  therefore  crowds  down- 
ward and  takes  the  place  of  the 
warmer  air.  Currents  are  pro- 
duced in  the  air  similar  to  those 
produced  in  water,  and,  as  a  re- 
sult of  the  currents,  the  entire 
air  finally  becomes  heated. 

Place  a  piece  of  sheet-iron 
on  a  ring  stand  over  a  Bunsen 
burner  (Fig.  72).  Hold  a  paper 
pin-wheel  over  the  sheet-iron, 
causes  the  wheel  to  rotate. 


T 
J 
/ 

*', 

ttV 

M  t 
^1 

uL_u5 

/    i 


FIG.  72. 


The  ascending  hot  air 
If  convenient,  hold  the  same 


136  ELEMENTAET  PHYSICS 

wheel  near  the  side  of  a  stove-pipe  and  notice  the  same 
result,  caused  in  this  case  by  the  ascending  currents  of 
heated  air  along-  the  sides  of  the  heated  stove-pipe. 

If  the  door  connecting-  a  warm  room  with  a  cold  one  is 
opened  slightly,  and  a  lighted  candle  is  held  at  the  base, 
the  coming  in  of  the  cold  air  is  readily  detected  by 
the  fact  that  the  flame  is  blown  inward.  If  the  candle  is 
held  near  the  top  of  the  door,  the  flame  is  blown  out- 
ward, showing  that  the  cold  air  is  passing  beneath  the 
warm  air,  and  that  warm  air  is  rising  above  the  cold  air. 

In  order  to  ventilate  a  room  during  winter  weather 
without  too  great  a  loss  of  heat,  the  lower  part  of  the 
window  should  be  raised.  In  order  to  cool  the  room 
as  rapidly  as  possible,  the  upper  window  should  be  low- 
ered. 

When  fire  is  placed  in  a  stove,  the  colder,  heavier  air  in 
the  room  pushes  the  much  warmer  and  lighter  air  in  the 
stove  up  the  chimney.  The  heated  air  is  not  drawn  up 
the  chimney  from  above  as  is  often  supposed,  but  is 
forced  up  by  the  weight  of  the  colder  air  surrounding  it. 

128.  Convection  Due  to  Displacement  of  Molecules. — Heat 
is  conveyed  to  the  more  remote  parts  of  liquids  and  gases 
by  means  of  currents.  The  molecules  in  contact  with  the 
source  of  heat  are  set  in  violent  vibration.  The  heated 
molecules  are  continually  pushed  aside,  and  give  the 
cooler  molecules  an  opportunity  to  become  heated  in 
their  turn.  The  final  result  is  that  all  the  molecules 
become  heated.  The  transference  of  heat  through  liquids 
and  gases,  by  means  of  a  circulation  among  the  mole- 
cules, is  called  convection. 

Why  the  molecules  of  liquids  and  gases  are  not  able  to 
communicate  their  vibrations  readily  from  molecule  to 
molecule,  as  is  the  case  with  many  solids,  is  unknown. 


HEAT  137 

The  result,  however,  is  that  the  transference  of  heat  in 
liquids  and  gases  is  dependent  upon  currents. 

129.  Sensation  of  Heat  and  Cold  cannot  be  Relied  upon  to 
Determine  Temperature. — Although  the  body  is  usually  able 
to  distinguish  between  heat  and  cold,  it  is  not  always  a  safe 
guide.     Take  three  basins,  the  first  containing  ice-water, 
the  second,  lukewarm  water,  and  the  third,  water  as  hot 
as  can  be  endured  without  injury.     Place  the  left  hand  in 
the  ice-water  and  the  right  hand  in  the  hot  water.     As 
soon  as  the  hands  have  grown  somewhat  accustomed  to 
the  temperatures  in  the  two  basins,  plunge  both  hands  at 
the  same  time  into  the  lukewarm  water.     The  left  hand 
now  feels  warm,  while  the  right  hand  feels  cool.     In  fact, 
the   left  hand  feels  warmer  than  the  right  hand,  not- 
withstanding the  fact  that  both  hands  are  in  the  same 
basin. 

During  winter  weather,  persons  coming  into  a  room 
from  the  very  cold  air  of  the  street,  often  call  the  air  in 
the  room  hot  and  close.  A  person  working  vigorously  in 
the  same  room  may  also  consider  the  air  too  warm.  Nev- 
ertheless a  thermometer  hanging  on  the  wall  may  regis- 
ter a  temperature  of  only  68°,  and  most  of  the  persons 
seated  in  the  room  may  consider  the  air  a  trifle  cool. 
A  thermometer  is  the  only  safe  guide  for  determining  the 
actual  temperature  of  a  room.  We  cannot  rely  alto- 
gether upon  our  senses. 

130.  The  Sensations  of  Temperature,  Pain,  and  Touch.— It 
is  commonly   stated  that  man  has  five  senses :    seeing, 
hearing,  tasting,  smelling,  and  feeling.     The  result  of  re- 
cent investigations,  however,  indicates  that  a  number  of 
distinct  sensations  have  been  usually  classed  together 
under  the  heading,  feeling.    Among  these  are  the  sensa- 
tions of  touch,  temperature,  and  pain. 


138  ELEMENTARY   PHYSICS 

Investigations  indicate  that  there  are  as  many  kinds  of 
nerves  as  there  are  sensations ;  that  each  kind  of  nervea 
is  so  constituted  as  to  be  particularly  sensitive  toward 
some  special  form  of  irritation.  Thus,  the  nerves  of  sight, 
if  irritated  by  a  pin,  are  not  aroused  to  a  true  sensation  of 
touch  or  pain. 

Each  kind  of  nerve  terminates  at  some  special  part  of 
the  body.  Thus,  the  nerves  of  sight  terminate  in  the  eye  ; 
those  of  hearing,  in  the  ear;  of  taste,  in  various  parts  of 
the  mouth ;  of  smell,  in  the  nose.  No  attempt  was  made 
formerly  to  distinguish  carefully  between  the  areas  in 
which  the  nerves  of  touch,  temperature,  or  pain  terminate. 
In  consequence  no  distinction  was  discovered  between 
these  three  different  kinds  of  nerves.  They  were  classed 
together  as  nerves  of  feeling,  and  it  was  believed  that  any 
part  of  the  skin  capable  of  feeling  one  of  these  sensations 
was  also  capable  of  feeling  the  other  two. 

However,  it  is  now  known  that  in  certain  diseases  of  the 
spinal  cord  areas  of  skin  may  be  mapped  out  in  which 
the  sensation  of  pressure  (touch)  is  lost,  while  that  of  tem- 
perature remains ;  areas  may  also  be  found  in  which  the 
sensation  of  temperature  is  lost  and  that  of  pressure  re- 
mains. Sometimes,  in  cases  of  persons  under  surgical 
treatment,  before  the  patient  comes  fully  under  the  influ- 
ence of  ether  or  of  chloroform,  the  sensation  of  touch  re- 
mains, while  that  of  pain  is  lost.  In  each  of  these  cases, 
the  facts  indicate  that  one  class  of  nerves  has  lost  its 
power  to  receive  sensations,  while  the  other  class  of  nerves 
mentioned  still  remains  active. 

Recent  investigations  indicate  that  there  are  two  sets 
of  nerves  of  temperature:  one  to  indicate  warmth,  the 
other  to  indicate  cold.  In  some  diseases  the  patient  can 
appreciate  warmth  applied  to  the  skin  but  not  cold.  The 


HEAT  139 

areas  of  the  skin  in  which  the  sensation  of  heat  is  percep- 
tible, and  the  areas  in  which  the  sensation  of  cold  can  be 
felt,  have  been  carefully  mapped  out  on  various  parts  of 
the  surface  of  the  body,  and  the  discovery  has  been  made, 
that  these  areas  are  often  distinct,  although  sometimes 
they  overlap.  These  areas  appear  to  be  the  places  where 
the  nerves  capable  of  perceiving  a  rise  of  temperature, 
and  those  capable  of  perceiving-  a  fall  of  temperature,  ter- 
minate. Since  the  sensations  of  heat  and  cold  are  often 
localized  in  different  areas,  it  seems  reasonable  to  con- 
clude that  the  sensations  are  felt  by  different  nerves. 

By  a  similar  method  of  investigation  it  has  been  deter- 
mined that  there  are  special  nerves  to  convey  the  sensa- 
tion of  pain. 

131.  Extremes  of  Temperature.— By  means  of  the  electric 
furnace,  M.  Moissan,  of  Paris,  has  secured  temperatures 
as  high  as  6,000  to  7,000  degrees  Fahrenheit.  The  tem- 
perature of  the  sun  is  considerably  greater.  Widely 
different  estimates  have  been  made.  Probably  16,000  to 
18,000  degrees  Fahrenheit  is  near  the  truth.  Owing  to 
the  scattering  of  the  heat  as  it  spreads  away  from  the  sun, 
only  one  two-billionth  of  the  total  quantity  of  heat  given 
out  by  the  sun  ever  reaches  the  earth.  The  remainder 
passes  to  other  points  in  space. 

When  liquid  air  is  allowed  to  evaporate,  temperatures 
of  about  312  degrees  below  zero,  Fahrenheit,  are  pro- 
duced. By  allowing  solid  hydrogen  to  liquefy  and  then 
to  evaporate,  Professor  Dewar,  of  London,  has  succeeded 
in  getting  temperatures  as  low  as  432  degrees  below  zero. 
Theoretically,  the  lowest  temperature  which  it  will  ever 
be  possible  to  secure  is  460  degrees  below  zero,  Fahren- 
heit. It  is  unknown  whether  there  is  any  upper  limit  to 
temperature. 


CHAPTEE  IV 
CHEMISTRY,   ATOMS 

132.  Principles  Underlying  Recognition  of  Gases. — Gases 
play  a  very  important  part  in  many  chemical  phenomena. 
Therefore,  the  ability  to  recognize  gases  is  of  the  highest 
importance.     Some  gases  may  be  distinguished  with  some 
exactness  by  their  color,  or  by  their  peculiar  odor.     But 
many  gases  are  both  colorless   and   odorless,  and  their 
recognition  depends  upon  a  clear  understanding  of  their 
chemical  properties.     Among  the  most  important  of  these 
colorless  gases  are  oxygen,  hydrogen,  carbon  dioxide,  and 
nitrogen. 

The  action  of  these  gases  has  been  studied  under  many 
kinds  of  chemical  influences,  and  the  action  of  each  gas 
under  each  of  these  influences  is  fully  known.  By  a 
comparison  of  the  results  obtained,  it  has  been  deter- 
mined which  actions  or  combinations  of  actions  are  char- 
acteristic of  each  particular  gas,  and  which  therefore  can 
be  used  as  a  means  of  identification.  Several  of  the  im- 
portant characteristics  of  oxygen,  hydrogen,  carbon  diox- 
ide, and  nitrogen  are  given  in  the  following  paragraphs. 

133.  Pneumatic   Trough.  —  Procure   a  galvanized  iron 
trough  (Fig.  73)  constructed  in  accordance  with  the  fol- 
lowing directions : — 

Height,  8  inches.  Width,  10  inches.  Length,  18  inches. 
Edges  along  the  top  strengthened  by  turning  them  over 
a  strong  wire.  A  handle  at  each  end  of  the  trough  for 

140 


CHEMISTEY,    ATOMS  141 

convenience  in  lifting.  Four  holes,  f  of  an  inch  in 
diameter,  near  the  upper  edge  of  the  trough  ;  each  hole 
3  inches  from  the  nearest  corner,  the  lower  edge  of  the 
hole  not  more  than  one  inch  below  the  top  of  the  trough. 
Across  the  centre  of  the  trough,  a  permanent  shelf,  also 
of  galvanized  iron,  strengthened  by  turning  the  edges 
over  strong  iron  wire  ;  this  shelf,  8  inches  wide,  so  sup- 
ported as  to  be  2  inches  below  the  top  of  the  trough ;  in 
this  shelf,  four  holes  f  of  an  inch  in  diameter,  each 


FXG.  73. 

hole  2  inches  from  the  edge  of  the  shelf  and  2.5  inches 
from  the  side  of  the  trough. 

Fill  the  trough  with  water.  Lower  a  fruit-jar  into  the 
water,  and  allow  the  water  to  rush  in  and  fill  the  jar  com- 
pletely, so  that  no  air  remains  in  the  jar.  This  can  be 
done  easily  if  jars  of  proper  size  and  form  are  selected. 
Invert  the  jar  in  the  water,  raise  it,  and  place  it  over  one 
of  the  holes  in  the  shelf.  If  the  mouth  of  the  jar  be  kept 
below  the  surface  of  the  water,  none  of  the  water  will  es- 
cape. It  is  held  up  by  the  pressure  of  the  air.  On  this 
account  the  trough  is  usually  called  a  pneumatic  trough. 

Instead  of  the  pneumatic  trough  here  described,  a  pan 


142  ELEMENTARY   PHYSICS 

or  basin  may  be  used.  Fill  the  jar  with  water,  cover  it 
with  a  piece  of  wet  pasteboard,  invert  it,  place  its  mouth 
beneath  the  level  of  the  water  in  the  basin,  remove  the 
pasteboard  and  support  the  jar  on  a  piece  of  the  hoop  of 
a  bucket  bent  into  the  form  of  a  letter  V  placed  horizon- 
tally. Insert  the  end  of  the  delivery  tube  (§  134)  beneath 
the  mouth  of  the  jar. 

134.  Method  of  Securing  Oxygen, — Crush  four  ounces  of 
potassium  chlorate  in  a  mortar,  until  it  is  about  as  coarse 
as  granulated  sugar.  Mix  it  thoroughly  with  four  ounces 
of  manganese  dioxide.  Place  in  a  te^t-tube  enough  of 
this  mixture  to  fill  one-third  of  the  tube  (Fig.  73).  Close 
the  top  of  the  tube  with  a  one-hole  rubber  stopper. 
Through  the  hole  insert  a  glass  tube,  and  over  the  glass 
tube,  slip  one  end  of  a  piece  of  rubber  tubing  about  30 
inches  long.  Thrust  the  free  end  of  this  rubber  tube 
through  the  nearest  opening  in  the  side  of  the  trough  and, 
leading  it  through  the  water,  push  it  up  through  that  hole 
in  the  shelf  which  is  directly  beneath  the  mouth  of  an  in- 
verted jar  filled  with  water.  The  friction  between  the 
rubber  tubing  and  the  rough  sides  of  the  opening  is 
sufficient  to  hold  the  tubing  in  place. 

Hold  the  test-tube  in  an  inclined  position,  and  heat  it 
by  means  of  a  Bunsen  burner.  At  first  apply  a  moderate 
amount  of  heat  to  the  upper  part  of  the  powder,  shifting 
the  flame  of  the  burner  back  and  forth,  so  as  not  to  injure 
the  test-tube  by  overheating  a  small  part  of  the  glass 
while  the  rest  of  the  tube  is  still  comparatively  cold.  In- 
crease the  heat  gradually  until  gas  is  given  off  freely,  but 
not  with  such  rapidity  as  to  visibly  disturb  the  powder. 
If  the  gas  is  given  off  too  rapidly,  hold  the  Bunsen  burner 
at  a  greater  distance.  As  the  supply  of  gas  given  off  by 
the  upper  part  of  the  powder  begins  to  lessen,  apply  the 


CHEMISTKY,    ATOMS  143 

heat  to  the  lower  part  of  the  tube.  Toward  the  end  of 
the  experiment  it  will  be  necessary  to  increase  the 
amount  of  heat  considerably  in  order  to  keep  up  the  free 
flow  of  the  gas.  This  gas  will  flow  through  the  rubber 
tube  and  will  escape  into  the  water  just  within  the  mouth 
of  the  jar  on  the  shelf  in  the  pneumatic  trough.  Do  not 
collect  the  gas  until  it  bubbles  freely  through  the  water. 
The  rubber  tube  used  to  convey  the  gas  is  called  the  de- 
livery tube. 

As  the  gas  escapes  from  the  tube,  bubbles  of  the  gas 
rise  to  the  top  of  the  inverted  jar  and  collect  there  in  such 
quantities  that  they  push  the  water  toward  the  lower 
part  of  the  jar.  As  the  gas  accumulates,  less  and  less 
water  is  left  in  the  jar.  Finally  gas  fills  the  entire 
jar  and  begins  to  escape  at  the  bottom.  Now  move  the 
jar  to  some  other  part  of  the  shelf,  keeping  the  mouth  of 
the  jar  below  the  surface  of  the  water.  Prepare  a  fresh 
jar  for  the  collection  of  an  additional  supply  of  gas. 
Jars  filled  with  oxygen  may  be  removed  without  losing 
any  of  the  gas  by  covering  the  mouth  of  the  jar  with  wet 
filter-paper  or  a  wet  blotter  before  lifting  it  out. 

As  soon  as  the  flame  of  the  Bunsen  burner  is  removed, 
the  gas  within  the  test-tube  cools,  shrinks  in  volume,  and 
tends  to  form  a  vacuum.  The  -pressure  of  the  outside  air 
is  then  likely  to  force  cold  water  through  the  delivery 
tube  into  the  hot  test-tube.  This  almost  invariably  cracks 
the  test-tube.  Therefore,  the  delivery  tube  should  always 
be  removed  from  the  pneumatic  trough  before  removing 
the  test-tube  from  the  flame  of  the  Bunsen  burner. 

135.  Properties  of  Oxygen.— Oxygen  is  slightly  heavier 
than  air.  It  may  therefore  be  kept  in  bottles  which  are 
not  inverted,  provided  the  mouths  are  covered  with  wet 
filter-paper.  When  retained  in  an  inverted  bottle,  the 


144  ELEMENTARY   PHYSICS 

mouth    must    dip  into  water,  the  latter  serving  as  a 
stopper. 

Set  fire  to  a  long-  splinter  of  wood.  Blow  out  the 
flame  and,  while  the  end  of  the  splinter  is  still  glowing-, 
thrust  it  down  into  a  jar  filled  with  oxygen.  The  glow- 
ing tip  will  burst  into  flame.  Repeat  the  operation  until 
the  supply  of  oxygen  is  exhausted.  Oxygen  supports 
combustion.  Heat  the  unwound  end  of  a 
stranded  iron  picture-wire  red  hot,  dip  it  into 
powdered  sulphur,  and,  while  the  sulphur  is 
still  burning,  lower  the  wire  into  the  jar  filled 
with  oxygen  (Fig.  74).  The  iron  burns  with 
brilliant  scintillations.  Drops  of  a  black  melted 
substance  fall  to  the  bottom  and  often  crack  the 
jar,  unless  the  bottom  of  the  jar  is  covered  by  at  least  an 
inch  in  depth  of  water.  A  piece  of  pasteboard  held  over 
the  top  of  the  jar  while  the  burning  is  taking  place,  will 
prevent  the  too  ready  escape  of  the  gas. 

If  a  burning  match  is  brought  in  contact  with  the 
oxygen  at  the  mouth  of  the  jar,  where  the  gas  comes  in 
contact  with  the  air,  the  oxygen  does  not  burn,  although 
the  match  burns  more  brightly.  Mixed  with  air,  it  does 
not  explode.  When  oxygen  is  mingled  with  lime-water, 
the  lime-water  remains  colorless  and  transparent. 

136.  Oxygen  is  Present  in  Ordinary  Air. — The  glowing  tip 
of  a  splinter  of  wood  will  not  burst  into  flame  on  being 
held  quietly  in  air,  but  it  may  be  caused  to  burst  into 
flame  by  bringing  it  in  contact  with  more  air,  either  by 
waving  the  splinter  through  the  air,  or  by  blowing  air 
against  the  glowing  end  of  the  splinter.  In  other  words, 
air  acts  as  though  it  contained  oxygen  diluted  with  con- 
siderable quantities  of  some  other  gas  which  does  not 
support  combustion. 


CHEMISTKY,   ATOMS 


145 


137.  Method  of  Securing  Hydrogen. — Introduce  into  a 
two-necked  Woulfe  bottle  sufficient  granulated  zinc,  or 
sheet  zinc  cut  into  small  pieces,  to  form 
a  layer  at  least  half  an  inch  deep  (Fig-.  75). 
Pour  in  water  enough  to  raise  the  water 
level  about  half  an  inch  above  the  zinc. 
In  one  of  the  necks  of  the  bottle  place  a 
rubber  stopper,  and  through  the  hole  in 
the  stopper  thrust  a  thistle-tube  until 
the  bottom  of  the  tube  is  below  the  sur- 
face of  the  water.  In  the  second  neck  of 
the  bottle  place  another  rubber  stopper. 
Through  the  hole  in  this  stopper  thrust 
one  end  of  a  bent  glass  tube ;  to  the 
other  end  of  this  tube  attach  the  de- 
livery tube.  Fill  with  water  two  jars 
on  the  shelf  of  the  pneumatic  trough, 
as  in  the  preceding  experiment,  and 
introduce  the  open  end  of  the  delivery 
tube  into  the  mouth  of  one  of  these 


jars. 


75. 


Pour  into  the  thistle-tube  a  mixture 
of  one  part  of  hydrochloric  acid  and  three  parts  of  water. 
Hydrogen  is  given  off  at  the  place  of  contact  between 
the  acid  and  the  zinc.  As  this  hydrogen  passes  over 
into  the  jar,  it  carries  with  it  the  air  which  is  in  the 
Woulfe  bottle.  The  first  jar  of  gas  collected  is  a  mixt- 
ure of  hydrogen  and  air.  Since  a  mixture  of  hydrogen 
and  air  explodes  violently  when  set  on  fire,  the  first  jar 
of  gas  collected  should  be  thrown  away  as  soon  as  the 
collecting  of  gas  in  the  second  jar  has  been  started. 
Of  course,  heat  should  not  be  applied  at  any  time  to 
the  Woulfe  bottle,  nor  should  any  flame,  large  or  small, 


146  ELEMENTARY   PHYSICS 

be  allowed  in  the  vicinity  of  the  apparatus  while  the 
collecting-  of  gases  is  going  on. 

In  order  to  ascertain  whether  the  hydrogen  is  pure,  be- 
fore the  collecting-  of  gas  in  the  second  jar  is  begun,  col- 
lect a  test-tube  full  of  the  gas  in  the  same  manner  as  in 
the  case  of  the  jars.  Raise  the  test-tube  from  the  water, 
being  careful  to  retain  its  inverted  position,  and  light  the 
gas  quickly  at  the  mouth  of  the  tube.  If  the  gas  burns 
quietly  and  without  any  explosion,  it  is  pure.  Since 
there  is  only  a  small  quantity  of  gas  in  the  test-tube,  the 
test  is  not  dangerous  even  if  the  hydrogen  is  still  mixed 
with  air. 

As  soon  as  the  production  of  gas  becomes  slow,  more 
hydrochloric  acid  should  be  added.  In  adding  the  acid, 
never  fill  more  than  half  of  the  bowl  of  the  thistle-tube  ; 
otherwise  any  unexpectedly  rapid  production  of  gas  may 
produce  such  a  great  pressure  within  the  bottle,  that  a 
part  of  the  acid  may  be  pushed  out  of  the  top  of  the 
thistle-tube. 

138.  Properties  of  Hydrogen, —Place  a  bottle  containing 
hydrogen  in  an  upright  position  and  remove  the  paper 
or  glass  covering  the  mouth,  for  three  minutes — by  the 
clock.  Drop  a  lighted  match  into  the  jar.  Why  has  the 
gas  escaped  ? 

Hydrogen  weighs  TV  as  much  as  air.  Therefore  in  the 
following  experiments  the  jars  or  test-tubes  containing 
the  hydrogen  should  be  held  in  a  vertical  position,  mouth 
downward. 

If  a  burning  splinter  of  wood  is  thrust  up  into  an  in- 
verted jar  filled  with  hydrogen,  the  flame  is  extinguished 
by  the  hydrogen  (Fig.  76).  Hydrogen  does  not  support 
combustion.  However,  the  gas  itself  is  set  on  fire  where 
it  comes  in  contact  with  the  air,  at  the  open  mouth  of  the 


CHEMISTRY,   ATOMS 


147 


FIG.  76. 


jar.  The  oxygen  in  the  air  supports  the  combustion  of 
the  hydrogen.  When  hydrogen  is  mingled  with  lime- 
water,  the  lime-water  remains  colorless  and 
transparent. 

If  the  hydrogen  in  the  jar  is  thoroughly 
mixed  with  air,  the  hydrogen  in  all  parts  of 
the  jar  can  burn  at  the  same  time,  and  if 
it  is  set  on  fire,  it  burns  so  rapidly  and  vig- 
orously that  violent  explosion  results.  On 
this  account  it  is  safer  not  to  use  a  fruit- jar 
full  of  hydrogen  for  these  experiments.  A 
large  test-tube  filled  with  hydrogen  is  suffi- 
cient for  most  experiments. 

139.  Method  of  Securing  Carbon  Dioxide. — 
In  a  two-necked  Woulfe  bottle  (Fig.  75)  place  about  one- 
third  of  a  tumblerful  of  marble  or  of  limestone  broken 
into  small  lumps.  Pour  in  water  enough  to  cover  the 
bottom  of  the  bottle.  Insert  a  thistle-tube,  connect  the 
delivery  tube,  and  arrange  for  the  escape  of  gas  into  the 
jars  on  the  shelf  of  the  pneumatic  trough,  as  in  the  pre- 
ceding experiments.  Then  pour  in  concentrated  hydro- 
chloric acid  enough  to  cover  the  marble.  In  order  to  se- 
cure carbon  dioxide  unmixed  with  air,  throw  away  the 
first  jarful  of  gas  collected. 

Since  carbon  dioxide  is  considerably  heavier  than  air,  it 
may  be  collected  without  the  use  of  the  pneumatic  trough. 
Through  the  mouth  of  an  erect,  empty  jar  thrust  the  open 
end  of  the  delivery  tube,  so  that  the  gas  will  escape  into 
the  jar  at  the  bottom.  The  air  in  the  jar  will  be  gradually 
pushed  out  by  the  entering  gas.  The  flow  of  the  gas 
should  be  allowed  to  continue  for  some  time,  in  order  to 
be  certain  that  the  last  remnant  of  air  has  been  removed 
from  the  jar.  In  order  to  retain  the  gas,  the  jar  should 


148 


ELEMENTARY   PHYSICS 


be  covered  with  a  piece  of  wet  filter-paper  or  window 
glass. 

At  the  ordinary  pressure  of  the  air,  water  absorbs  its 
own  volume  of  carbon  dioxide.  If  it  be  desirable  to  re- 
tain for  any  length  of  time  the  gas  collected  in  the  jar 
over  the  pneumatic  trough,  slip  a  piece  of  ordinary 
window  glass  under  the  mouth  of  the  jar,  and  set  the  jar 
aside  in  an  erect  position. 

140.  Properties  of  Carbon  Dioxide. — Since  carbon  dioxide 
weighs  1J  times  as  much  as  air,  the  jar  containing  this 
gas  should  be  placed  in  an  erect  position.  If  a  burning 


FIG.  77. 

splinter  of  wood  is  thrust  down  into  a  jar  filled  with  car- 
bon dioxide,  the  flame  is  extinguished,  and  the  carbon 
dioxide  does  not  ignite  at  the  mouth  of  the  jar,  where  it 
comes  in  contact  with  the  air.  It  does  not  explode  when 
mingled  with  air. 

Pour  carbon  dioxide  down  a  slanting  wooden  trough, 
along  which  is  arranged  a  row  of  short,  lighted  candles 
(Fig.  77).  The  candles  are  extinguished  in  succession,  as 
soon  as  they  are  covered  by  the  down  ward- flowing  but 
invisible  gas. 

Pour  water  into  a  fruit- jar  containing  carbon  dioxide 


CHEMISTRY,   ATOMS  149 

until  it  is  one-third  full  of  water.  Close  the  jar  air-tight, 
shake  it  vigorously,  and  open  it  again  under  water,  mouth 
downward.  A  large  part  of  the  carbon  dioxide  has  dis- 
appeared. It  has  been  taken  up  or  dissolved  by  the 
water. 

When  the  gas  is  allowed  to  flow  for  a  short  time 
through  lime-water,  the  lime-water  assumes  a  milky  ap- 
pearance. The  same  effect  may  be  produced  by  pouring 
a  small  quantity  of  lime-water  into  a  jar  containing  car- 
bon dioxide  and  shaking  the  jar  violently.  In  conse- 
quence of  the  shaking  the  gas  and  the  lime-water  be- 
come intermingled. 

In  order  to  prepare  lime-water,  place  a  small  lump  of 
quick-lime,  such  as  is  used  by  plasterers  and  masons,  in 
a  jar  full  of  water.  Allow  the  jar  to  stand  over  night, 
then  pour  off  the  clear  liquid  and,  if  it  is  not  perfectly 
clear,  filter  it. 

141.  Carbon  Dioxide  Present  in  Ordinary  Air  and  in  the 
Breath. — Place  lime-water  in  a  bottle  closed  with  a  rub- 
ber stopper  provided  with  two  openings  (Fig.  78). 
Through  one  opening 
thrust  a  glass  tube  be- 
neath the  surface  of  the 
lime-water,  until  near 
the  bottom  of  the  bottle. 
Through  the  other  open- 
ing thrust  a  short  tube 
extending  only  a  short 

distance    beneath    the 

_  FIG.  78. 

stopper.      Through    the 

second  tube  suck  the  air  out  of  the  upper  part  of  the 
bottle.  Ordinary  air  runs  down  the  first  tube  and  bub- 
bling up  through  the  lime-water  gradually  produces  a 


150 


ELEMENTARY  PHYSICS 


milky  appearance,  thus  showing  the  presence  of  small 
quantities  of  carbon  dioxide  in  ordinary  air. 

Eeplace  the  lime-water  with  a  fresh  solution  and,  re- 
versing the  process,  blow  the  breath  through  the  first 
tube.  The  milky  appearance  is  pro- 
duced much  more  rapidly,  thus  show- 
ing that  the  lungs  give  out  a  greater 
quantity  of  carbon  dioxide  than  they 
take  in.  In  other  words,  the  lungs 
increase  the  quantity  of  carbon  dioxide 
in  the  air. 

142.  Method  of  Securing  Nitrogen. — 
Take  a  glass  tube  about  18  inches  long, 
and  having  an  internal  diameter  of 
about  half  an  inch.  Round  the  edges 
at  the  ends  by  holding  them  a  short 
time  in  the  flame  of  a  Bunsen  burner 
and,  after  cooling,  close  both  ends  of 
the  tube  by  means  of  rubber  stoppers. 
With  a  file  mark  off  transverse  lines  at 
points  3,  6,  9, 12  and  15  inches  from  the 
end  of  the  stopper,  closing  the  upper 
end  of  the  tube  (Fig.  79).  This  will 
leave  a  space  of  more  than  2  inches 
B  between  the  last  mark  and  the  stopper 
at  the  lower  end  of  the  tube.  Fill  this 
space  with  the  dark-brown  liquid 
known  as  potassium  pyrogallate  and 
again  close  the  tube.  Air  now  occu- 
pies a  length  of  15  inches  (Fig.  79  A).  Shake  the  liquid 
back  and  forth  in  the  tube  for  at  least  15  minutes.  Then 
open  the  lower  end  of  the  tube  beneath  the  water  sur- 
face. Water  rushes  up  into  the  tube,  showing  that  a 


Fro.  79. 


CHEMISTEY,   ATOMS  151 

part  of  the  air  has  been  dissolved  by  the  potassium 
pyrogallate. 

In  the  meantime  a  large  part  of  the  potassium  pyro- 
gallate,  since  it  is  a  heavier  liquid  than  water,  escapes 
into  the  water  beneath.  Close  the  tube  and  shake  it 
back  and  forth  again.  Most  of  the  potassium  pyrogallate 
is  now  loosened  from  the  walls  of  the  tube  and  sinks 
with  the  water  into  the  lower  part  of  the  tube.  Open 
the  lower  end  of  the  tube  again  below  the  water  surface, 
and  raise  or  lower  the  tube  until  the  water  in  the  tube 
and  in  the  vessel  are  at  the  same  level  (Fig.  79  B).  Then 
close  the  tube  again.  That  part  of  the  air  which  remains 
in  the  tube  now  occupies  only  -$•  as  much  space  as  the 
space  originally  occupied  by  the  air.  It  moreover  ex- 
hibits entirely  different  properties.  It  is  that  part  of  air 
known  as  nitrogen. 

143.  Presence  of  Nitrogen  in  the  Air. — Thrust  a  burning 
splinter  of  wood  into  that  part  of  the  air  remaining  in 
the  tube  after  the  preceding  experiment.  The  flame  is 
extinguished  at  once.  It  is  evident  that  the  oxygen 
which  was  formerly  present  in  the  air  has  been  removed 
by  means  of  the  potassium  pyrogallate.  Moreover  the 
experiment  enables  us  to  determine  the  relative  propor- 
tion of  oxygen  in  ordinary  air.  Since  ^  of  the  ordinary 
air  has  disappeared  at  the  close  of  the  experiment,  this 
suggests  that  oxygen  forms  about  ^  of  the  air.  It  was 
seen  in  a  former  experiment  that  the  proportion  of  car- 
bon dioxide  in  ordinary  air  is  small,  in  fact,  only  about 
^Vtf  of  any  voltftne  of  ordinary  air  consists  of  carbon 
dioxide.  Very  small  quantities  of  other  gases  are  also 
present.  Among  these  argon  has  recently  attracted  con- 
siderable attention.  The  gas  which  remains  in  the  tube 
in  the  preceding  experiment,  however,  is  nearly  all  ni- 


152 


ELEMENTARY  PHYSICS 


trogen.     Nitrogen,  therefore,  forms  about  f  of  ordinary 
air. 

144.  Properties  of  Nitrogen. — Objects  placed  in  nitrogen 
do  not  burn.     On   the   contrary,  nitrogen   extinguishes 
burning  objects.     Under   ordinary  conditions  it   is  im- 
possible   to    set    fire    to  nitrogen.      Mingled  with   air, 
it  does  not  explode.     Mingled  with  lime-water,  it  does 
not  produce  a  milky  appearance.     Under  ordinary  condi- 
tions nitrogen  is  a  very  inactive  or  inert  gas.     It  may  be 
distinguished  from  oxygen,  hydrogen,  and  carbon  dioxide 
by  the  fact  that  it  does  not  produce  the  effects  produced 
by  these  gases.     This  inactivity  of  nitrogen  is  its  most 
striking  characteristic.     It  weighs  slightly  less  than  air. 

145.  Tests  for  Oxygen,   Hydrogen,    Carbon  Dioxide,  and 
Nitrogen. — The  various  properties  of  the  gases  so  far  dis- 
cussed may  be  recorded  in  the  form  of  the  following 
table. 


Table  indicatingthe 
most  prominent 
tests  of  several 
colorless  gases. 

Weight  of  the 
gas  compared 
with  weight 
of  air. 

Causes  a  glow- 
ing splinter  of 
wood  to  burst 
into  flame. 

Enables  metals 
to  burn. 

Burns  when  ig- 
nited where  it 
comes  in  con- 
tact with  air. 

Explodes  when 
mixed  with 
a  i  r  and  ig- 
nited. 

Causes  a  white 
precipitate  in 
lime-water. 

Extinguishes 
burning  sab- 
stances. 

4-1 

+ 

-\- 

Hydrogen 

1  0 

Tv 

+ 

+ 

+ 

Carbon  Dioxide 

I 

-f 

-\- 

Nitrogen 

T97T7n 

-+- 

From  this  table  it  may  be  seen  which  tests  are  most 
suitable  for  the  rapid  identification  of  any  of  these  gases, 
provided  no  other  gases  are  present.  If  the  gas  is  color- 
less and  odorless,  the  bursting  into  flame  of  the  glowing 
tip  of  a  splinter  of  wood  introduced  into  the  gas  indi- 


CHEMISTRY,   ATOMS  153 

cates  the  presence  of  oxygen.  The  explosion  of  the  gas 
when  it  is  mingled  with  air  and  set  on  fire  suggests 
hydrogen.  The  production  of  a  milky  appearance,  when 
the  gas  is  passed  through  lime-water,  identifies  it  as 
carbon  dioxide.  The  failure  of  all  three  of  these  tests 
suggests  nitrogen. 

A  much  larger  number  of  gases  is  known  to  chemists. 
Sometimes  two  or  three  kinds  of  tests  must  be  used 
before  one  of  these  gases  is  fully  identified. 

146.  Apparatus  used  to  Determine  the  Composition  of 
Water. — Cut  off  the  bottom  from  a  large  wide-mouthed 
bottle.  If  no  glass  cutter  is  at  hand,  this  may  be  accom- 
plished in  the  following  manner.  With  a  new  three- 
cornered  file  produce  a  sharp,  deep  scratch,  about  half 
an  inch  long,  near  the  bottom  of  the  bottle.  Heat  the 
end  of  a  poker,  or  of  an  iron  rod  a  little  thicker  than  a 
lead  pencil,  until  it  is  red  hot,  then  allow  it  to  cool,  until 
the  red  color  has  disappeared.  Hold  the  heated  end  of 
the  poker  against  one  end  of  the  scratch  for  about  four 
seconds.  Reverse  the  position  of  the  poker  and  hold  it 
against  the  other  end  of  the  scratch  for  about  eight 
seconds.  Bring  it  back  to  its  original  position  and  hold 
it  there  until  the  glass  cracks.  As  soon  as  the  glass 
cracks,  draw  the  poker  along  the  surface  of  the  bottle, 
keeping  it  about  one-eighth  of  an  inch  ahead  of  the 
crack.  The  crack  will  slowly  follow  the  poker  around 
the  bottle.  After  the  bottom  of  the  bottle  has  been  cut 
off,  the  sharp  edges  can  be  removed  with  a  file. 

Secure  a  cork  large  enough  to  close  the  mouth  of  the 
bottle  (Fig.  80).  Solder  two  thin  strips  of  platinum  2J 
inches  long  and  £  inch  wide,  to  two  strong  copper  wires. 
Thrust  the  wires  down  through  the  cork  as  far  as  the 
platinum  strips,  at  such  distances  apart  that  it  is  possible 


154 


ELEMENTARY   PHYSICS 


to  slip  two  test-tubes  into  the  cut  end  of  the  bottle  and 
over  the  platinum  strips.  Cover  the  upper  side  of  the 
cork  and  the  connection  between  the 
copper  wire  and  the  platinum  strips 
thoroughly  with  sealing-wax.  Insert 
the  cork  in  the  bottle  and  add  sealing- 
wax  around  the  edge  of  the  cork,  so 
as  to  form  a  water-tight  vessel,  into 
which  the  platinum  strips  project 
from  beneath.  If,  on  trial,  the  vessel 
permits  a  little  water  to  escape,  dry 
thoroughly  before  adding  any  more 
sealing-wax,  otherwise  the  wax  will 
not  adhere  to  the  glass. 

Apparatus  for  use  in  this  experi- 
ment, often  more  complicated,  is  sold 
by  dealers  under  the  name  of  Appa- 
ratus for  the  Electrolysis  of  Water. 
147.  Composition  of  Water. — Fasten  the  bottle  on  a  ring 
stand  and  nearly  fill  it  with  water.  Fill  two  long  test- 
tubes  or  combustion-tubes  with  water.  Close  the  mouth 
of  each  test-tube  with  the  thumb  or  with  a  rubber  stop- 
per, invert  the  tube,  place  the  end  just  closed  under  the 
surface  of  the  water  in  the  bottle,  and  then  remove  the 
thumb  or  stopper.  The  pressure  of  the  air  will  hold  in 
the  water.  Slip  the  mouth  of  each  test-tube  over  one  of 
the  platinum  strips  (Fig.  80),  and  fasten  the  test-tubes  in 
a  vertical  position  so  that  the  tubes  cannot  move  when 
filled  with  gas.  Remove  water  from  the  jar,  using  a 
rubber  tube  as  a  siphon,  until  it  is  certain  that  if  the 
water  in  the  test-tube  were  added  to  the  water  in  the  jar 
there  would  be  no  overflow.  If  the  water  already  con- 
tains acid,  fill  the  rubber  tube  with  water ;  close  both 


Fro. 


CHEMISTRY,   ATOMS  155 

ends  with  fingers,  insert  one  end  in  the  water  in  the 
jar,  lower  the  other  end  and  remove  the  fingers. 

To  the  copper  wires  projecting  beneath  the  cork  attach 
the  terminal  wires  coming  from  an  electric  battery  or  dy- 
namo. For  the  electric  battery,  six  potassium  bichromate 
cells  (Fig.  128)  will  be  sufficient.  For  the  student  un- 
acquainted with  electrical  phenomena,  it  is  sufficient  to 
know  that  the  purpose  of  the  battery  or  dynamo  is 
simply  to  send  a  current  of  electricity  through  the 
water. 

As  soon  as  the  electricity  begins  to  flow,  gas  bubbles 
up  at  the  surface  of  both  of  the  platinum  strips  and  col- 
lects at  the  top  of  the  test-tubes,  pushing  out  the  water. 
The  rapidity  of  the  accumulation  of  gas  may  be  very  much 
increased  by  mixing  sulphuric  acid  with  the  water  in  the 
proportion  of  about  one  to  ten.  After  the  flow  of  elec- 
tricity has  continued  for  some  time,  it  is  seen  that  the 
quantity  of  gas  collected  in  one  of  the  test-tubes  is  twice 
as  great  as  that  collected  in  the  other. 

The  gases  are  cojorless  and  their  identity  can  be  estab- 
lished only  by  experiment.  Allow  the,  gases  to  collect 
until  the  tube  containing  the  greater  quantity  of  gas  is 
full.  Remove  the  tube,  keeping  it  in  its  inverted  position, 
and  thrust  up  into  its  mouth  a  burning  splinter.  The 
flame  of  the  splinter  is  extinguished,  but  the  gas  burns 
where  it  comes  in  contact  with  the  air.  Blow  out  the 
flame.  Mix  the  gas  with  air  and  bring  a  lighted  match 
near  the  mouth  of  the  tube.  The  mixture  explodes.  The 
quantity  of  gas  used  is  usually  too  small  to  break  the 
tube.  Hold  the  tube  by  means  of  a  handkerchief  wrapped 
around  its  upper  end  if  you  have  any  reason  to  expect  a 
stronger  explosion.  The  gas  is  hydrogen. 

By  the  time  the  identity  of  the  more  rapidly  accumu- 


156  ELEMENTARY   PHYSICS 

lating  gas  has  been  determined,  the  other  test-tube  will 
probably  be  full.  Hold  the  test-tube  with  the  mouth  up- 
ward and  thrust  the  glowing  tip  of  a  splinter  of  wood 
down  into  the  gas.  The  tip  bursts  into  flame.  The  gas  is 
oxygen. 

Only  hydrogen  is  given  off  in  one  of  the  test-tubes,  and 
only  oxygen  collects  in  the  other.  If  there  were  a  mixt- 
ure of  these  gases  in  either  one  of  the  tubes,  there  would 
be  an  explosion  when  a  burning  splinter  is  thrust  in,  al- 
though the  violence  of  the  explosion  would  depend  upon 
the  proportion  of  each  gas  in  the  mixture.  No  other  gas 
except  hydrogen  and  oxygen  can  be  discovered.  At  the 
close  of  the  experiment,  nothing  is  left  in  the  bottle  ex- 
cept the  sulphuric  acid  which  was  added  at  the  beginning 
of  the  experiment,  and  the  water  which  has  not  yet  been 
separated  into  hydrogen  and  oxygen.  In  other  words, 
the  experiment  demonstrates  that  water  consists  of  a  com- 
bination of  hydrogen  and  oxygen. 

Electricity  has  the  power  not  only  of  separating  water 
into  its  component  gases,  but  also  of  collecting  the  gases 
at  separate  points.  The  oxygen  collects  at  the  platinum 
strip  (  +  )  by  means  of  which  the  current  of  electricity 
enters  the  water,  and  the  hydrogen  collects  at  the  strip 
(— )  by  means  of  which  the  current  of  electricity  leaves 
the  water. 

148.  Relative  Proportion  of  Hydrogen  and  Oxygen  in  Wa- 
ter.— It  is  evident  that  twice  as  great  a  volume  of  hydro- 
gen is  produced  as  of  oxygen.  If,  however,  these  quanti- 
ties of  gas  are  weighed  (  §  2 )  it  is  found  that  the  one 
volume  of  oxygen  weighs  8  times  as  much  as  the  two 
volumes  of  hydrogen.  Any  volume  of  oxygen,  therefore, 
weighs  16  times  as  much  as  an  equal  volume  of  hydro- 
gen. 


CHEMISTRY,   ATOMS  157 

The  composition  of  water  may  be  represented  as  fol- 
lows: 

Water  =  Hydrogen  +  Oxygen. 
(18)  (2)  (16) 

The  volumetric  composition  may  be  suggested  in  the 
following  manner : 


Water  = 

(Hydrogen)          (Oxygen) 

149,  Formation  of  Water  from  Oxygen  and  Hydrogen. — If 

two  volumes  of  hydrogen  and  one  volume  of  oxygen  be 
placed  in  a  vessel  and  an  electric  spark  be  sent  through 
the  mixture,  the  gases  will  unite  and  form  water  ( §  173). 

The  production  of  water  from  hydrogen  and  oxygen 
may,  however,  be  shown  in  a  much  more  simple  form. 
Attach  a  platinum  tipped  nozzle  ( §  36)  to  the  end  of  a 
delivery  tube  connected  with  an  apparatus  for  the  secur- 
ing of  hydrogen  (§  137).  Collect  a  test-tube  full  of 
hydrogen,  and  thrust  into  it  a  burning  splinter.  If  there 
is  no  explosion,  there  is  no  admixture  of  air  with  the 
hydrogen  coming  from  the  apparatus.  Then,  and  not  till 
then,  is  it  safe  to  light  the  gas  escaping  from  the  platinum 
nozzle.  A  very  dangerous  and  violent  explosion  may  occur 
if  this  precaution  is  not  observed.  The  danger  may  be  en- 
tirely removed  by  placing  the  Woulfe  bottle  in  a  cage 
constructed  of  coarse  wire-gauze,  permitting  only  the 
thistle-tube  and  the  delivery  tube  to  protrude.  Any  tin- 
ner can  construct  such  a  cage.  In  case  of  explosion  the 
broken  glass  is  then  caught  by  the  wire-gauze. 

After  the  hydrogen  has  been  ignited  hold  the  nozzle 
within  the  upper  part  of  a  cold  bell-jar,  but  not  near 


158  ELEMENTARY   PHYSICS 

enough  to  crack  the  jar  (Fig.  81).  Water  collects  on  the 
sides  of  the  interior  of  the  jar. 

A  little  thought  will  show  that  something  unusual  has 
taken  place.  Heat  does  not  ordinarily  cause  water  to 

settle  on  objects.  On  the  con- 
trary, heat  is  used  to  drive  off 
water  which  has  settled  on 
any  surface,  by  causing  it  to- 
evaporate. 

But    when   the   hydrogen 
burns,  it  unites  with  the  oxy- 
FJG  81  gen    in    the    air,    and    forms 

water.     This  water  settles  on 

the  glass.  Directly  over  the  flame  the  heat  usually  causes 
the  water  to  evaporate  as  rapidly  as  it  settles  there,  and 
the  upper  part  of  the  bell -jar,  therefore,  usually  remains 
practically  dry.  On  the  sides  of  the  bell- jar,  however, 
the  water  collects  more  rapidly  than  it  evaporates,  and 
hence  the  sides  of  the  jar  are  soon  covered  with  a  thin 
mist,  or  even  with  large  drops  of  water. 

150.  Different  Action  of  Oxygen  in  Air  and  Oxygen  in 
Water. — Oxygen  in  air  shows  all  of  the  properties  of  pure 
oxygen,  with  only  this  difference, — its  action  is  relatively 
weaker.  This  is  due  solely  to  the  fact  that,  in  air,  the 
oxygen  is  diluted  with  four  times  its  volume  of  nitrogen, 
a  very  inactive  gas.  (§§  136,  143.) 

Oxygen  in  water,  however,  shows  entirely  different 
qualities  from  pure  oxygen.  Thrust  a  burning  stick  into 
pure  oxygen  or  into  air  and  it  continues  to  burn.  Thrust 
it  into  water  or  steam  and  it  is  extinguished.  Air  is  the 
great  supporter  of  combustion.  Water  is  the  great  enemy 
of  combustion,  the  most  commonly  used  fire  extinguisher. 
Moreover,  the  hydrogen  with  which  oxygen  is  com- 


CHEMISTRY,    ATOMS  159 

bined  in  water,  is  not  an  inactive  substance  like  nitrogen. 
It  lias  already  been  shown  that  a  mixture  of  hydrogen 
and  oxygen  brought  in  contact  with  fire  explodes  vio- 
lently, and  that  hydrogen  in  contact  with  air  will  burn. 
In  fact  the  heat  produced  by  the  union  of  hydrogen 
and  oxygen  is  the  greatest  heat  produced  outside  of  the 
electric  furnace.  Neither  the  oxygen  nor  the  hydrogen 
•  when  combined  in  the  form  of  water  act  as  these  gases 
usually  do  either  when  separate  or  mixed. 

151.  Mixtures,  Compounds,   and   Components. — The   dif- 
ferent action  of  oxygen  in  air  and  in  water  is  explained 
by  the  fact  that  air  is  merely  a  mixture,  and  each  of  the 
gases  in  the  mixture  retains  its  own  characteristic  prop- 
erties.    Water,  however,  is  not  a  mixture  of  two  gases, 
but  a  new  substance  formed  from  these  gases,  possessing 
none  of  the  properties  of  the  gases  from  which  it  is  formed. 
A  new  substance  formed  by  the  union  of  two  or  more  dif- 
ferent substances,  and  having  properties  dissimilar  from 
those  of  any  of  the  components,  is  called  a  compound.    Hy- 
drogen and  oxygen  are  therefore  components  of  the  com- 
pound^ water.     They  are  said  to  be  chemically  united,  not 
mixed,  and  the  force  which  causes  the  gases  to  unite  is 
called  chemical  affinity.   These  expressions  are  often  found 
convenient.     It  is  well,  however,  to  remember  that  to 
have  a  name  for  a  process  is  not  equivalent  to  explaining 
its  nature.     Although  much  has  been  learned  about  the 
conditions  under  which  substances  unite,  and  about  the 
proportions  in  which  they  combine,  practically  nothing 
is  known  as  yet  of  the  real  nature  of  the  force  called 
chemical  affinity. 

152.  Analysis  and  Synthesis. — The  composition  of  a  com- 
pound substance  may  be  determined  either  by  separating 
the  substance  into    its   components,  or  by  taking  the 


160  ELEMENTARY   PHYSICS 

necessary  components  and,  by  a  chemical  union  of  these 
components,  producing  the  compound  substance  whose 
composition  it  is  desired  to  know.  The  first  process  is 
called  Analysis  (loosening-  the  components),  the  second 
process  is  called  Synthesis  (putting  together  the  com- 
ponents). 

In  the  preceding  paragraphs  the  composition  of  water 
was  determined  by  both  analysis  (§  147)  and  synthesis 
(§  149).  There  are  several  methods  of  analysis.  The 
separation  of  the  components  by  means  of  an  electric  cur- 
rent may  be  called  the  electrolytic  method.  The  applica- 
tion of  a  considerable  quantity  of  heat  at  high  tempera- 
ture is  often  capable  of  driving  the  components  of  a 
compound  apart.  This  method  of  separation  may  be 
called  the  thermal  method  of  analysis.  The  red  oxide  of 
mercury  may  be  divided  into  its  components  by  this 
method.  In  order  to  recognize  its  components,  it  is 
necessary  to  be  able  to  identify  both  oxygen  and  mercury 
readily. 

153.  The  Properties   of  Mercury. — When  mercury  falls 
upon  the  floor  it  usually  breaks  into  countless  fragments 
of  all  sizes,  and  each  fragment,  large  or  small,  assumes  the 
form  of  a  more  or  less  flattened  sphere.    When  mercury 
is  heated  gently  at  the  bottom  of  a  test-tube,  it  evapo- 
rates and  collects  again  at  the  top  of  the  tube,  where  the 
glass  is  still  comparatively  cold.     Often,  along  the  top 
of  the  tube,  it  forms  a  bright  coating,  called  a  mercury 
mirror.     A  strip  of  copper  wire,  dipped  into  mercury  and 
rubbed,  assumes  a  bright,  silvery  appearance. 

154.  The  Composition  of  Red  Oxide  of  Mercury. — Place  a 
small  quantity  of  red  oxide  of  mercury  (a  powder)  in  a 
test-tube.     Close   the   mouth  of  the  tube  with  a  rubber 
stopper  through  which  passes  a  glass  tube  connecting 


CHEMISTRY,   ATOMS 


161 


with  the  delivery -tube  (Fig-.  82).  Weigh  the  test-tube 
with  the  stopper  and  powder.  Connect  the  delivery-tube 
with  the  test-tube  and  pneumatic  trough,  as  in  the  appa- 
ratus for  the  collection  of  oxygen  (§  134),  but,  for  the  sake 
of  economy,  use  a  wide  8-inch  test-tube  for  the  collection 
of  any  gas  that  may  be  given  off.  Hold  the  test-tube  in  a 
horizontal  position,  and  tap  it  gently  so  as  to  spread  out 
the  powder.  Heat  the  powder,  being  careful  to  avoid 
heating  intensely  any  single  small  spot  of  the  test-tube. 
Gas  begins  to  escape  from  the  delivery-tube  and  bubbles 


FIG.  82 

up  through  the  water.  The  first  portion  of  the  gas  is 
simply  the  air  which  filled  the  tube  at  the  beginning  of 
the  experiment  and  which  is  expanded  by  heat.  This  air 
should  be  thrown  away.  The  volume  of  the  gas  thrown 
away  should  not  be  much  greater  than  the  total  volume 
contained  in  the  delivery -tube  and  in  the  test-tube  hold- 
ing the  powder.  As  the  test-tube  becomes  hotter,  gas  is 
given  off  from  the  oxide  of  mercury  and  collects  in  the 
8-inch  test-tube.  As  soon  as  the  disengagement  of  gas 
slackens,  take  the  delivery-tube  out  of  the  water,  taking 
care  that  no  water  remains  in  the  end  of  the  tube.  Then, 


162  ELEMENTARY   PHYSICS 

and  not  till  then  (§  134),  remove  the  flame  from  the  test- 
tube. 

Slip  a  piece  of  wet  filter  paper  under  the  mouth  of  the 
test-tube  containing  the  gas  and  turn  the  test-tube  into  an 
upright  position.  Remove  the  filter  paper  and  thrust  the 
glowing  end  of  a  splinter  of  wood  down  into  the  tube. 
The  glowing  tip  bursts  into  flame.  This  indicates  that 
the  gas  is  oxygen. 

Weigh  the  test-tube  containing  the  powder,  together 
with  its  contents,  and  the  stopper,  as  soon  as  they  have 
cooled  to  the  temperature  of  the  room.  There  has  been 
a  distinct  loss  in  weight.  This  is  due  to  the  loss  of  the 
oxygen  which  was  driven  off  from  the  powder  by  the 
heat. 

The  material  remaining  in  the  test-tube  is  now  not  all 
red  oxide  of  mercury.  In  the  upper  part  of  the  tube  are 
found  minute  globules  and  also  a  coating  of  a  bright  me- 
tallic-looking liquid.  In  order  that  this  metallic  coating 
may  not  be  driven  off  by  the  heat,  the  upper  part  of  the 
test-tube  must  not  be  heated.  Remove  the  stopper.  In- 
vert the  test-tube  and  tap  its  mouth  vigorously  on  the 
open  palm  of  the  hand.  The  metallic  liquid  forms  a 
large  drop  in  the  palm  of  the  hand.  The  drop  is  proba- 
bly coated  with  some  of  the  red  oxide  of  mercury  from 
which  the  oxygen  has  not  been  driven  off.  This  coating 
can  be  easily  removed  with  the  finger.  A  piece  of  clean 
copper  wire,  if  dipped  into  the  drop  and  then  rubbed, 
assumes  a  bright,  silvery  appearance.  If  allowed  to 
fall,  the  drop  breaks  up  into  smaller  drops  of  various 
sizes,  and  even  the  smallest  drop  takes  the  form  of  a 
sphere.  The  metallic  liquid  is  unquestionably  mercury. 

If  the  red  oxide  of  mercury  is  heated  long  enough,  it 
completely  disappears.  Nothing  but  mercury  remains  in 


CHEMISTRY,   ATOMS  163 

the  test-tube,  and  the  only  substance  which  has  left  the 
test-tube  is  the  oxygen,  of  which  there  may  be  a  suffi- 
cient quantity  to  fill  a  small-sized  jar.  The  red  oxide  of 
mercury  therefore  consists  of  a  combination  of  oxygen 
and  mercury.  This  fact  was  utilized  in  inventing  a  name 
for  the  substance. 

155.  The  Relative  Proportion  of  Mercury  and  Oxygen  in  the 
Red  Oxide  of  Mercury.  — If  the  weight  of  the  mercury  pro- 
duced from  any  given  quantity  of  the  red  oxide  of  mer- 
cury is  compared  with  the  weight  of  the  oxygen,  it  is 
found  that  the  mercury  weighs  exactly  12J  times  as  much 
as  the  oxygen. 

At  ordinary  temperatures  there  is  a  great  difference  in 
the  volumes  occupied  by  the  mercury  and  the  oxygen  ob- 
tained from  the  red  oxide  of  mercury,  for,  at  ordinary 
temperatures,  mercury  is  in  a  liquid  form,  and  oxygen  is 
in  a  gaseous  form.  The  volume  of  the  oxygen,  therefore, 
greatly  exceeds  that  of  the  mercury,  in  fact,  about  10,104 
times  at  68°  F. 

When  both  are  in  the  gaseous  state  the  difference  in 
volume  is  much  less.  If  mercury  is  sufficiently  heated  it 
turns  into  vapor.  At  high  temperatures  both  mercury 
and  oxygen  are  in  the  gaseous  condition.  If,  while  both 
are  in  the  gaseous  condition,  at  the  same  temperature  (for 
instance  700°  F.)  and  under  the  same  pressure,  the  mer- 
cury and  oxygen  given  off  by  any  quantity  of  red  oxide 
of  mercury  be  compared,  the  volume  occupied  by  the 
mercury  will  be  found  to  be  exactly  twice  the  volume  oc- 
cupied by  the  oxygen. 

This  is  another  illustration  of  the  fact  that  substances 
combine  in  very  simple  proportions,  if  the  proportions 
are  based  upon  the  volumes  of  these  substances  while  in  a 
gaseous  condition  (§  148). 


164  ELEMENTARY   PHYSICS 

The  composition  of  the  red  oxide  of  mercury  may  be 
represented  as  follows : — 

Red  Oxide  of  Mercury  _  Mercury        Oxygen 

(13.5)  (12.5)  (1) 

The  volumetric  composition  may  be  indicated  in  the 
following  manner : 


Red  Oxide  of  Mercury   = 


(Mercury)    (Oxygen) 

156.  All  Components  of  a  Compound  not  Always  Readily 
Detected. — In  the  preceding  experiments,  illustrating  the 
electrolytic  and  thermal  methods  of  analysis,  it  was  pos- 
sible to  recognize  readily  both  components  of  the  com- 
pounds examined.  In  many  compounds  it  is  possible  to 
identify  only  one  component  with  ease. 

If,  for  instance,  a  solution  of  copper  sulphate  is  poured 
into  the  bottle  prepared  for  the  electrolysis  of  water 
(§  146),  and  a  current  of  electricity  is  sent  through  the  solu- 
tion, copper  is  deposited  on  one  of  the  platinum  strips. 
The  only  source  from  which  the  copper  could  have  come 
is  the  copper  sulphate,  since  the  tops  of  the  copper  wire 
holding  the  platinum  strips  are  buried  beneath  the  seal- 
ing wax.  Copper  is,  therefore,  one  of  the  components  of 
copper  sulphate.  The  other  components  undergo  chemi- 
cal changes  not  here  explained.  They  remain  dissolved 
in  the  water,  and  since  they  possess  no  distinctive  color 
they  cannot  be  seen. 

Instead  of  using  the  electrolysis  apparatus,  wires  from 
the  battery  or  dynamo  may  be  connected  to  two  electric- 
light  carbons,  and  the  ends  of  these  carbons  may  be 
dipped  into  the  copper  sulphate.  When  a  current  is  sent 
through  the  solution,  the  copper  is  deposited  on  one  of 


CHEMISTRY,   ATOMS  165 

the  carbons.  The  copper,  adheres  better  to  the  platinum 
or  carbon,  if  it  is  deposited  slowly  by  a  weak  current  of 
electricity. 

If  the  current  of  electricity  is  allowed  to  flow  until  all 
the  copper  in  the  copper  sulphate  solution  has  settled 
upon  one  of  the  platinum  strips  or  electric-light  carbons, 
the  weight  of  the  copper  is  found  to  be  exactly  ^\  or  f  J 
of  the  entire  weight  of  the  copper  sulphate  crystals  origi- 
nally placed  in  the  water. 

Copper  Sulphate  =  Copper  +  Other  Components 
(249)  (63)  (186) 

157.  Substitution,  or  the  Replacement  of  one  Component 
of  a  Compound  by  another  Substance.— The  presence  of 
copper  in  copper  sulphate  may  be  demonstrated  by  a 
method  which  does  not  require  the  application  of  either 
electricity  or  heat. 

Pour  a  small  quantity  of  a  solution  of  copper  sulphate 
into  a  test-tube.  Place  a  large  bright  wire  nail  in  the 
tube.  Close  the  tube  with  the  thumb  and  repeatedly  tip 
it,  so  that  the  solution  covers  the  nail  and  recedes  again. 
Copper  settles  on  the  iron. 

If  the  nail  is  taken  out  of  the  test-tube  as  soon  as  the 
copper  covering  is  distinctly  seen  (a  few  seconds  are 
usually  enough)  and  if  the  coating  is  first  allowed  to 
dry  and  is  then  vigorously  rubbed  with  cloth,  the  character- 
istic color  of  smooth  copper  may  be  recognized.  The  other 
components  of  copper  sulphate,  however,  remain  in  the 
solution,  as  in  the  case  of  the  preceding  experiment,  and, 
since  they  exhibit  no  distinctive  coloring,  their  presence 
again  escapes  detection. 

If  the  nail  is  allowed  to  remain  in  the  copper  sulphate 
for  a  long  time,  the  copper  coating  increases  in  thickness, 


166  ELEMENTARY  PHYSIOS 

but  it  no  longer  adheres  well  to  the  nail  and  may  easily 
be  rubbed  off.  If  now  the  nail  is  closely  examined,  it  is 
discovered  that  the  surface  is  distinctly  roughened,  owing 
to  the  disappearance  of  that  part  of  the  iron  which  once 
formed  the  smooth  surface  of  the  nail.  The  pitted  char- 
acter of  the  surface  may  be  distinctly  recognized  under  a 
lens,  and  a  comparison  with  another  nail  of  the  same 
kind  shows  that  its  diameter  has  been  perceptibly  dimin- 
ished. The  iron  disappears  while  the  quantity  of  copper 
which  settles  on  the  nail  increases.  The  iron  takes  the 
place  of  the  copper  in  the  copper  sulphate  solution. 

Copper  has  an  attraction  for  the  other  components  of 
copper  sulphate.  From  the  experiment  it  appears  that 
iron  also  has  an  attraction  for  these  components.  In  fact, 
it  seems  to  have  a  stronger  attraction  for  them  than  does 
copper.  The  iron  therefore  pushes  the  copper  away  from 
the  other  components  of  copper  sulphate  and  takes  its 
place.  In  other  words,  iron  is  substituted  for  copper  in 
copper  sulphate  and  it  forms  a  new  substance,  called  iron 
sulphate.  The  copper  which  is  pushed  aside  settles  upon 
the  iron.  nail. 

If  enough  iron  is  placed  in  the  copper  sulphate  solu- 
tion to  replace  all  of  the  copper,  the  color  of  the  solution 
gradually  changes  from  the  blue  of  copper  sulphate  to 
the  green  of  iron  sulphate.  If  the  excess  of  iron  with 
its  copper  coating  is  taken  out  and  the  water  is  removed 
by  heating  the  solution  in  an  evaporating  dish,  the  iron 
sulphate  is  left  behind  and  may  easily  be  recognized  as 
something  quite  distinct  from  copper  sulphate.  The 
chemical  change  may  be  expressed  as  follows : 

[Copper + Other  components] + Iron  =  [Iron -+-  Other  components] + Copper 

(63)  (186)  (55.6)    (55.6)  (186)  (63) 

(Copper  sulphate)  +  Iron  =  (Iron  sulphate)  +-  Copper 


CHEMISTRY,   ATOMS  167 

This  is  one  of  the  simplest  illustrations  of  the  identi- 
fication of  one  component  of  a  compound  by  chemical 
substitution. 

158.  Identification  of  Components  of  Compounds  by  Color 
of  Precipitates. — In  the  cases  of  analysis  of  compounds 
so  far  discussed,  the  components  which  were  identified 
were  secured  free  from  union  with  any  other  substance. 
It  is  often  possible  to  recognize  the  presence  of  one  or 
more  of  the  components  of  a  compound  without  securing1 
them  in  the  free  state,  in  other  words — without  obtaining 
them  uncombined  with  any  other  substance. 

Dissolve  about  1.7  of  an  ounce  of  potassium  bichro- 
mate in  one  pound  of  water.  Mingle  a  small  quantity  of 
nitric  acid  with  4  times  its  volume  of  water.  Pour  a*  small 
part  of  the  nitric  acid  solution  into  a  test-tube  and  add  a 
little  of  the  potassium  bichromate  solution.  Nothing  is 
noticed  beyond  the  mingling  of  the  two  liquids. 

In  another  test-tube  place  a  small  piece  of  silver  (coin), 
add  dilute  nitric  acid  and  heat  for  a  short  time  in  the 
flame  of  a  Bunsen  burner.  A  part  of  the  silver  disap- 
pears in  the  acid  solution.  Pour  a  little  of  the  acid  solu- 
tion into  a  third  test-tube,  dilute  it  still  farther  with  water 
(pure  distilled  water),  and  again  add  a  little  of  the  potas- 
sium bichromate  solution.  The  mixture  becomes  clouded 
with  a  dark  red  substance,  which  evidently  can  be  due 
only  to  the  presence  of  silver  in  some  form  in  the  acid 
solution.  Most  of  this  dark  red  substance  promptly  set- 
tles to  the  bottom  of  the  test-tube.  This  portion  consists 
of  the  heavier  particles.  The  lighter  particles  remain 
suspended  for  a  long  time  in  the  liquid,  but  finally  these 
also  are  found  at  the  bottom  of  the  tube.  Hence  the  dark 
red  substance  is  called  a  precipitate. 

Now,  chemists  have  tested  the  effect  of  pouring  potas- 


168  ELEMENTARY   PHYSICS 

slum  bichromate  into  the  solutions  of  all  sorts  of  sub- 
stances whose  compositions  are  known,  and,  while  they 
have  succeeded  in  getting  precipitates  in  some  of  these 
cases,  the  color  of  none  of  these  precipitates  was  dark 
red,  excepting  in  those  cases  where  silver  was  present  in 
some  form  or  other  in  the  solution.  Having  once  estab- 
lished this  fact,  the  presence  of  a  dark  red  precipitate  on 
the  addition  of  potassium  bichromate  to  the  solution  of 
any  unknown  substance  is  at  once  an  evidence  of  the 
presence  of  silver  in  this  substance. 

Chemicals  which  are  used  for  the  purpose  of  ascertain- 
ing the  presence  of  various  substances  in  compounds  are 
often  spoken  of  as  reagents.  Potassium  bichromate  is  only 
one  of  a  great  many  reagents  used  to  produce  precipitates. 

159.  Several  Tests  Often  Necessary  to  Determine  Def- 
initely the  Presence  of  a  Substance  in  a  Compound. — 
Potassium  bichromate  produces  precipitates  only  when 
one  of  the  following  substances  is  present  in  the  unknown 
compound  which  is  in  solution :  lead,  silver,  mercury, 
bismuth,  and  barium.  The  presence  of  any  precipitate  on 
the  addition  of  potassium  bichromate  to  the  solution  of 
an  unknown  compound,  therefore,  is  at  once  an  evidence 
of  the  presence  of  one  or  more  of  these  five  substances  in 
the  compound.  However,  since  many  of  these  precipi- 
tates are  yellow,  the  formation  of  a  precipitate  on  the 
addition  of  potassium  bichromate  is  not  sufficient  to  de- 
termine which  metal  is  present  in  the  compound. 

But  other  reagents  also  form  precipitates  and  the  color 
of  these  precipitates  varies  so  much,  that  it  is  often 
possible,  if  several -re agents  are  used  in  succession,  to  de- 
termine with  confidence  the  presence  of  a  substance  in 
an  unknown  compound  even  in  cases  when  the  use  of  a 
single  reagent  leads  to  no  definite  result. 


CHEMISTRY,   ATOMS 


169 


This  fact  is  shown  by  the  following-  table.  In  the  first 
column  are  placed  the  names  of  the  different  substances 
which  give  rise  to  precipitates  when  the  compounds  con- 
taining1 them  are  dissolved  in  water,  and  when  some  one 
of  the  three  reagents  named  at  the  head  of  the  following- 
columns  is  added-  Opposite  the  names  in  the  first  col- 
umn are  indicated  the  colors  of  the  precipitates  formed 
on  the  addition  of  these  reag-ents.  When  no  precipitates 
are  formed,  the  space  is  left  blank. 


COMPOUNDS  CONTAIN- 

REAGENTS. 

TO  BE  TESTED. 

Potassium     Bichro- 
mate. 

Potassium  Iodide. 

Hydrochloric 
Acid. 

Lead. 

Yellow 

Bright  yellow 

White. 

Silver 

Dark  red 

Yellow-white,  darken- 

White, 

Mercury  

Yellowish    green 

ing  on  exposure. 
Greenish     yellow    to 

blacken- 
ing on  ex- 
posure. 
White. 

Bismuth 

to  red. 
Orange  yellow. 

red. 
Dark  brown 

Copper.  . 

White    to    yellow    to 

Antimony  

brownish  yellow. 
Yellow. 

Barium 

Light  yellow. 

Palladium  . 

Black. 

Suppose  potassium  bichromate  is  added  to  the  solution 
of  some  unknown  compound  and  a  yellowish  precipitate 
is  formed.  If  neither  potassium  iodide  nor  hydrochloric 
acid  form  a  precipitate  when  added  to  a  fresh  quantity  of 
the  same  unknown  solution,  barium  is  probably  pres- 
ent in  the  compound.  If  potassium  iodide  gives  a  dark 
brown  precipitate,  bismuth  is  one  of  the  components.  In 
this  case,  hydrochloric  acid  should  produce  no  precipi- 
tate. If  potassium  iodide  produces  a  yellow  precipitate, 
the  unknown  compound*  contains  lead.  In  this  case  hy- 


170  ELEMENTARY   PHYSICS 

drochloric  acid  produces  a  white  precipitate.  If  a  small 
quantity  of  potassium  bichromate  produces  a  yellowish 
green  precipitate,  and  if  a  greater  quantity  of  the  reagent 
produces  a  distinctly  greenish  or  reddish  precipitate, 
mercury  is  present.  Suppose  the  addition  of  potassium 
bichromate  produces  a  reddish  precipitate  of  such  a  tint 
that  it  is  impossible  to  determine  whether  silver  or  mer- 
cury is  present.  The  addition  of  potassium  iodide  should 
at  once  decide  this  matter. 

160.  Method  of  Separation  of  Different  Precipitates  Formed 
by  the  Same  Reagent. — "When  a  compound  or  a  mixture 
of  compounds  contains  several  substances  which  will 
give  precipitates  with  the  same  reagent,  it  is  necessary 
to  separate  these  substances  before  their  identity  can 
be  fully  established.  This  is  especially  true  when  the 
different  precipitates  have  about  the  same  color.  This 
separation  may  often  be  easily  accomplished  if  the  differ- 
ent precipitates  vary  considerably  as  to  their  solubility 
in  the  different  reagents.  This  may  be  illustrated  by  the 
following  example. 

When  dilute  hydrochloric  acid  is  added  to  all  known 
solutions  of  compounds,  precipitates  are  formed  only 
when  lead,  silver,  and  mercury  are  present.  Hence  the 
formation  of  a  precipitate  on  the  addition  of  hydrochloric 
acid  is  at  once  an  evidence  of  the  presence  of  either  lead, 
silver,  or  mercury,  or  of  any  two  of  these  substances, 
or  of  all  three  of  them.  Since  all  of  these  precipitates 
are  white,  it  is  impossible  to  determine  from  the  color 
alone  which  substance  or  substances  are  present. 

The  precipitates  differ,  however,  considerably  in  the 
readiness  with  which  they  dissolve  in  various  substances, 
and  this  fact  may  be  utilized  in  the  identification  of  the 
metal  present  in  the  unknown  compound.  For  instance, 


CHEMISTRY,   ATOMS  171 

the  precipitate  due  to  the  presence  of  lead  dissolves 
readily  in  hot  water,  but  those  due  to  the  presence  of 
silver  and  mercury  do  not  dissolve  in  hot  water.  The 
precipitate  due  to  the  presence  of  silver  dissolves  in  warm 
dilute  ammonia  water,  while  the  precipitate  due  to  the 
presence  of  mercury  turns  black  and  does  not  dissolve. 
The  following  method  of  procedure  will  make  it  possi- 
ble to  determine  which  substances  caused  the  precipi- 
tates due  to  the  addition  of  hydrochloric  acid. 

Place  a  filter  paper  in  a  funnel  supported  on  a  ring 
stand.  Put  a  beaker  directly  beneath,  and 
pour  the  precipitates  with  the  liquid  in 
which  they  were  formed  into  the  funnel. 
The  precipitates  remain  on  the  filter  pa- 
per; the  liquids  (filtrates)  pass  through 
and  escape  at  the  bottom.  Pour  water 
gently  over  the  precipitates  until  they 
are  thoroughly  clean.  Then  punch  a  hole 
through  the  bottom  of  the  filter  paper, 
and,  with  a  little  water,  wash  the  precip- 
itates into  a  second  small  glass  beaker. 
Add  water  to  the  precipitates,  place  the 
beaker  on  a  wire  gauze  on  a  ring  stand  (Fig.  83)  and  boil 
for  a  short  time.  If  all  of  the  precipitate  disappears, 
only  lead  is  present. 

If  all  of  the  precipitate  does  not  disappear  after  boil- 
ing in  plenty  of  water,  filter  mixture  while  hot.  If  any 
lead  is  present,  white  needle-shaped  crystals  will  appear 
in  the  filtrate  on  cooling.  The  precipitate  remaining  on 
the  filter  paper  contains  either  silver  or  mercury,  or  both. 

Wash  the  precipitate  left  on  the  second  filter  paper 
into  another  beaker,  add  ammonia  water,  and  warm  the 
mixture.  If  only  silver  is  present,  all  of  the  precipitate 


172  ELEMENTAEY  PHYSICS 

disappears.  If  the  precipitate  turns  black,  mercury  is 
present.  In  order  to  determine  whether  silver  is  present 
in  addition  to  the  mercury,  filter  the  black  mixture  and 
add  dilute  nitric  acid  to  the  filtrate,  until  the  filtrate 
changes  the  color  of  litmus  paper  from  blue  to  red.  If  a 
white  precipitate  appears,  silver  is  present. 

Lead  is  first  separated  from  silver  and  mercury,  and 
then  silver  is  separated  from  mercury.  After  the  separa- 
tion, the  other  tests  with  potassium  bichromate  and  po- 
tassium iodide  may  be  applied. 

161.  Elements. — The  preceding-  paragraphs  by  no  means 
describe  all  of  the  methods  employed  by  chemists  to  de- 
termine the  components  of  a  compound,  but  they  at  least 
make  evident  that  there  are  methods  for  attacking  such 
problems. 

Chemists  have  been  at  work  for  many  years  in  deter- 
mining the  components  of  all  kinds  of  compounds,  and, 
wherever  possible,  in  determining  even  the  components 
of  these  components.  Whenever  none  of  the  methods  at 
the  command  of  a  chemist  enable  him  to  subdivide  any 
component  into  two  or  more  other  components,  the  last 
component  obtained  is  called  an  dement.  It  is  impossi- 
ble, of  course,  to  determine  definitely  what  substances 
are  elements,  since  new  methods  of  analysis  may  in  the 
future  enable  the  chemist  to  subdivide  those  which  are 
now  universally  considered  as  elements.  But  it  is  easily 
possible  to  determine  what  substances  have  not  yet 
been  subdivided  and  must  therefore  be  considered  ele- 
ments for  the  present. 

An  element  is  a  substance  which  cannot,  by  any  method 
at  present  known  to  chemists,  be  divided  into  two  or  more 
dissimilar  substances. 

Nearly  80  substances  are  at  present  considered  as  ele- 


CHEMISTRY,   ATOMS  173 

ments  (§  181).  Most  of  these  are  known  only  to  the  chem- 
ist, since  they  rarely  exist  uncombined  in  nature.  Among 
the  most  commonly  known  elements  are  the  metals  : 
aluminum,  copper,  gold,  iron,  lead,  mercury,  nickel, 
platinum,  silver,  tin  and  zinc.  Brass  is  not  an  element 
but  a  mixture  chiefly  of  copper  and  zinc.  Bronze  is  a 
mixture  of  copper,  tin  and  zinc.  Arsenic,  carbon,  phos- 
phorus and  sulphur  are  elements.  The  gases,  hydrogen, 
nitrogen,  and  oxygen  are  elements.  Some  elements  are 
exceedingly  common  in  nature,  but  are  seen  uncombined 
with  other  elements  in  chemical  laboratories  only. 
Among  these  are  silicon  and  calcium.  Aluminum  has 
only  recently  become  one  of  those  metals  commonly  seen 
outside  of  the  chemical  laboratory.  In  fact,  until  re- 
cently it  was  a  very  expensive  metal,  and  difficult  to  ob- 
tain. 

162.  Chief  Elements  Present  in  the  Earth.— Nearly  50 
per  cent,  of  the  solid  crust  of  the  earth  consists  of  oxygen, 
and  a  little  more  than  25  per  cent,  consists  of  silicon,  so 
that  the  non-metallic  elements,  oxygen  and  silicon,  form 
nearly  75  per  cent,  of  the  crust  of  the  earth  ;  while  almost 
all  of  the  remainder  is  formed  by  the  metals:  aluminum, 
8  per  cent.  ;  iron,  5  per  cent. ;  calcium,  4  per  cent. ;  mag- 
nesium, 2.5  per  cent.  ;  sodium,  2.5  per  cent.,  and  potas- 
sium, 2.5  per  cent.     The  other  rock-forming  elements — 
titanium,  carbon,  hydrogen,  phosphorus,  manganese,  sul- 
phur, and  chlorine — form  scarcely  1  per  cent,  of  the  crust. 

The  rock-forming  compounds  are  usually  known  as 
minerals.  Compounds  or  mixtures  of  compounds  which 
are  commercially  valuable  as  sources  of  metals  are  usu- 
ally called  ores. 

163.  Chief  Elements  Found  in  Plants. — Water  forms  by 
far  the  largest  part  of  most  plants.     While  green  they 


174  ELEMENTARY  PHYSICS 

contain  from  75  to  91  per  cent,  of  water.     Even  so-called 
dry  wood  contains  usually  15  per  cent,  of  water. 

After  the  water  present  in  plants  has  been  completely 
evaporated,  there  remain,  in  a  dried  condition,  those 
compounds  which  once  formed  the  more  solid  parts  of 
the  roots,  stems,  leaves,  flowers,  and  fruits.  These  or- 
ganic compounds  consist  chiefly  of  carbon,  oxygen,  hy- 
drogen, and  nitrogen.  If  these  compounds  are  'burned, 
the  gaseous  substances  due  to  combustion  escape,  and 
only  the  mineral  constituents  remain.  The  mineral  con- 
stituents, called  ash  when  left  as  the  result  of  burning, 
form  by  far  the  smallest  part  of  plants.  Usually  less 
than  3  per  cent,  of  a  plant  remains  in  the  form  of  ash, 
and  many  juicy  plants  contain  less  than  1  per  cent,  of 
mineral  material.  In  the  ash,  potassium,  calcium,  mag- 
nesium, and  phosphorus  are  always  present  in  an  appre- 
ciable amount.  Iron,  chlorine,  sulphur,  and  sodium  are 
also  present,  but  usually  in  very  small  quantities.  Sili- 
con occurs  in  various  grasses,  sedges,  rushes,  and  other 
plants.  Iodine  is  found  in  some  sea  plants. 

164.  The  Chief  Elements  Present  in  the  Animal  Body, — 
It  has  been  calculated  that  in  an  average  man  weighing 
154  pounds,  116  pounds  consist  of  water  which  may  be 
driven  off  as  moisture,  and  29  pounds  consist  of  the  flesh, 
skin,  blood,  fat,  and  gelatine  after  all  the  water  has  been 
driven  off.  If  the  body  were  burned,  the  water  and  the 
organic  compounds  would  disappear  and  only  the  min- 
eral matter  would  be  left  behind.  This  would  consist 
chiefly  of  what  was  left  of  the  bones,  a  total  of  only  about 
10  pounds. 

Oxygen  forms  about  72  per  cent,  of  the  human  body  ; 
carbon,  13.5  ;  hydrogen,  9.1 ;  nitrogen,  2.5  ;  calcium,  1.3  ; 
phosphorus,  1.15 ;  sulphur,  1.47 ;  sodium,  .1 ;  chlorine, 


CHEMISTliY,    ATOMS  175 

.085 ;  fluorine,  .08  ;  potassium,  .026  ;  iron,  .01 ;  magnesium, 
.0012 ;  and  silicon,  .0002.  The  great  quantity  of  oxygen 
and  hydrogen  is  largely  due  to  the  abundance  of  water  in 
all  the  tissues.  Many  organic  compounds  in  the  body 
contain  sulphur.  Calcium  and  phosphorus  are  among 
the  most  important  constituents  of  bones.  Sodium  and 
chlorine  are  present  in  the  form  of  salt  dissolved  in  the 
various  liquids  of  the  body. 

165.  Organic  Compounds.— Plants  and  animals  are  able 
to  produce  many  compounds  not  otherwise  found  in  nat- 
ure.    Among  these  may  be  mentioned,  sugar  starch,  oil, 
wood,  fat,  muscle,  and  bone.     About  75  years  ago  it  was 
believed  that  chemists  never  would  be  able  to  duplicate 
any  of  these  compounds,  and  hence  a  very  careful  distinc- 
tion was  made  between  the  compounds  not  produced  by 
living  organisms,  and  those  which  could  be  produced  only 
by  plants  and  animals.     The  first  were  called  inorganic 
compounds,   and  the   second,   organic    compounds.      At 
present,  however,  many  of  the  so-called  organic  com- 
pounds, such  as  sugar  and  starch,  can  be  produced  in  the 
laboratory. 

166.  The   Chief  Difference  between  Animals  and  Plants. 
— If  the  student  is  acquainted  with  only  the  larger  animals 
and  plants,  he  will  not  find  it  difficult  to  find  many  dis- 
tinctions between  these  two  divisions  of  the  living  world. 
If  he  knows  also  the  microscopic  animals  and  plants,  he 
will  find  that  many   distinctions  which   can  readily  be 
made  between  the  larger  forms  cannot  be  retained  as  a 
means  of  separation  between  all  of  the  smaller  animals 
and  plants.     For  instance,  it  was  once  supposed  that  ani- 
mals are  capable  of  motion  and  plants  are  not.     But  now 
it  is  known  that  various  parts  of  plants  are  in  continual, 
although  very  slow,  motion ;  some  sensitive  plants  close 


176  ELEMENTARY   PHYSICS 

their  leaves  as  soon  as  touched;  some  insect-catching 
plants  close  their  leaves  too  rapidly  for  the  insects  to 
escape;  and  some  microscopic  plants  in  some  stages  of 
their  existence  can  swim  around  freely  in  the  water  even 
in  a  direction  opposite  to  that  of  the  current. 

There  is,  however,  one  difference  which  will  serve  bet- 
ter as  a  distinction  between  animals  and  plants  than  any 
of  the  rest.  Animals  can  not  manufacture  organic  com- 
pounds from  inorganic  substances.  They  must  obtain 
substances  which  are  already  organic  compounds,  and 
they  use  these  to  manufacture  from  them  new  organic 
compounds.  Plants,  however,  can  manufacture  organic 
compounds  from  inorganic  materials.  Plants,  therefore, 
can  live  on  the  inorganic  materials  they  secure  from  the 
earth,  the  water,  and  the  air;  but  animals  must  live  on 
plants,  or  on  animals  which  have  eaten  plants. 

167.  Water  and  Air  are  the  Sources  of  the  Chief  Con- 
stituents of  Plants  and  Animals.— The  chief  constituents 
of  animal  and  vegetable  bodies,  aside  from  water,  are  the 
carbon  compounds.  In  addition  to  carbon,  most  of  these 
compounds  contain  hydrogen  and  oxygen,  and  often  also 
nitrogen,  all  of  which  are  non-metallic  elements.  In 
marked  contrast  with  the  comparatively  small  import- 
ance of  carbon,  hydrogen,  and  nitrogen  in  rock-forming 
minerals,  is  their  great  importance  in  animal  and  vegetable 
compounds. 

A  large  part  of  the  hydrogen  and  oxygen  used  by 
plants  is  secured  from  the  water  which  is  present  in  the 
soil.  Both  animals  and  plants  secure  oxygen  also  from 
the  air,  the  process  being  called  breathing.  Nitrogen 
forms  four-fifths  of  the  air,  but  plants  take  in  nitrogen 
by  way  of  the  roots.  Carbon  united  with  oxygen,  in  the 
form  of  carbon  dioxide  ( §  141),  is  present  as  a  very 


CHEMISTRY,   ATOMS  177 

minor  constituent  of  the  air,  but  this  small  quantity  is  of 
the  greatest  importance,  since  it  provides  most  of  the 
carbon  used  by  plants.  The  supply  of  carbon  dioxide  in 
the  air  is  continually  replaced  by  the  decay  of  animal  and 
vegetable  bodies. 

Animals  secure  carbon  and  nitrogen  by  eating  plants 
or  animals  that  have  eaten  plants. 

168.  Elements  are  Usually  Found  Combined  in  Nature 

Most  elements  unite  so  readily  with  other  elements  that 
their  natural  condition  may  be  said  to  be  a  state  of  com- 
bination. Even  some  of  those  elements  which  are  found 
free  in  nature  occur  more  frequently  combined  than  un- 
combined.  This  is  true,  for  instance,  of  oxygen,  carbon, 
and  sulphur.  Nitrogen  is  also  frequently  found  in  com- 
bination in  the  organic  compounds  present  in  animal 
and  vegetable  bodies. 

Some  elements  can  unite  with  nearly  all  of  the  other 
elements.  This  is  true  to  a  remarkable  degree  of  oxygen, 
chlorine,  and  sulphur.  Other  elements,  like  gold,  appear 
to  unite  with  but  few  other  substances.  For  this  reason, 
gold  is  often  found  as  a  pure  metal  in  the  form  of  nug- 
gets. Elements  which  combine  with  difficulty  with  other 
elements  are  said  to  be  inert.  Very  few  elements  are 
inert.  If  an  element  is  desired  in  the  free  uncombined 
state,  it  is  usually  necessary  to  secure  some  compound  in 
which  the  element  is  present,  and  then  by  some  means 
separate  the  element  from  the  rest  of  the  compound,  in 
such  a  manner  that  the  element  will  remain  separated 
from  all  other  substances.  Some  elements  are  so  active 
that  special  precautions  must  be  used  in  order  to  retain 
them  in  the  free  state.  Phosphorus  must  be  kept  under 
water.  Potassium  and  sodium  must  be  kept  under  naph- 
tha or  benzene. 


178 


ELEMENTARY  PHYSICS 


169.  The  Formation  of  Compounds  by  Direct  Combination, 
—Since  a  state  of  combination  is  the  natural  condition 
for  most  elements,  it  is  a  comparatively  easy  matter  to 
illustrate  the  various  methods  of  combination. 

The  direct  combination  of  two  elements  is  shown  when 
hydrogen  is  burnt  in  air.  It  unites  with  oxygen  and 
forms  water  (§  149).  Chlorine  is  a  poisonous  gas,  not  suit- 
able for  experiment  in  an  ordinary  classroom  not  pro- 
vided with  special  apparatus  for  carrying  off  obnoxious 
gases.  However,  it  may  be  interesting  to  know  that  if 


FIG.  84. 

equal  volumes  of  chlorine  and  hydrogen  are  placed  in 
direct  sunlight,  they  unite  so  rapidly  as  to  cause  a  vio- 
lent explosion.  The  compound  formed  is  hydrochloric 
acid. 

When  iron  is  burnt  in  oxygen  (§  135),  a  black  compound, 
black  iron  oxide,  is  produced.  Red  hot  carbon  (charcoal) 
placed  in  oxygen  will  burn  and  produce  carbon  dioxide. 

If  a  current  of  chlorine  is  passed  over  iron  filings  heated 
in  a  hard  glass  tube  (Fig.  84  C),  the  chlorine  and  iron 
unite  with  such  rapidity  that  the  iron  becomes  white  hot. 


CHEMISTRY,   ATOMS  179 

Iron  chloride  is  the  result.  To  secure  the  chlorine,  place 
in  a  flask,  A,  a  third  of  an  ounce  of  manganese  dioxide, 
and  an  ounce  of  common  hydrochloric  acid,  and  gently 
heat  the  mixture.  To  dry  the  gas,  pass  it  through  a  tube 
filled  with  lumps  of  calcium  chloride,  B. 

Mix  thoroughly  j-0  of  an  ounce  of  fine  iron  filings  with 
J5  of  an  ounce  of  sulphur.  Place  about  one-fourth  of  the 
mixture  in  a  test-tube,  and  heat  until  the  mixture  glows. 
The  sulphur  unites  with  the  iron,  producing  a  bright 
light  and  forming  iron  sulphide. 

Mix  thoroughly  ^  of  an  ounce  of  coarsely  powdered 
sulphur  with  ^  of  an  ounce  of  copper  filings.  The  mixt- 
ure presents  neither  the  lemon-yellow  color  of  sulphur 
nor  the  well-known  color  of  copper.  The  sulphur  and 
copper,  however,  have  not  united.  If  the  mixture  be  ex- 
amined under  the  microscope,  the  particles  of  sulphur 
may  still  be  recognized  among  the  particles  of  copper. 
With  the  exercise  of  sufficient  care  and  diligence,  the 
particles  of  sulphur  may  be  picked  out  from  the  mixture, 
and  thus  may  be  separated  from  the  copper.  If  the  mixt- 
ure be  thrown  into  water,  the  copper  will  sink  to  the  bot- 
tom at  once,  while  the  sulphur,  which  weighs  less  than 
one-fourth  as  much,  will  sink  more  slowly  through  the 
liquid.  This  method  may  also  be  used  for  the  separation 
of  the  sulphur  from  the  copper 

Put  the  mixture  into  a  tube  of  hard  glass  (test-tube  or 
ignition-tube),  and  heat  it  with  a  Bunsen  burner.  The 
sulphur  melts,  catches  fire,  and  unites  with  the  copper, 
producing  a  brilliant  light.  When  no  further  change 
takes  place,  allow  the  tube  to  cool,  break  it,  and  examine 
the  contents.  Sulphur  and  copper  have  disappeared  as 
such.  A  new  substance  has  appeared,  which,  though  con- 
taining both  the  sulphur  and  the  copper,  has  no  external 


180  ELEMENTARY   PHYSICS 

resemblance  to  either,  and  possesses  new  properties.    It 
is  the  chemical  compound,  copper  sulphide. 

170.  Formation  of  New  Compounds  by  Substitution.— 
Many  compounds  are  formed,  not  by  direct  union  of  the 
elements,  but  by  substituting-  some  other  element  for 
one  of  the  elements  of  a  compound,  thus  forming  a  new 
compound.  Hydrochloric  acid  consists  of  hydrogen  and 
chlorine  (§  169).  When  zinc  is  placed  in  hydrochloric 
acid  it  takes  the  place  of  the  hydrogen,  and  forms  zinc 
chloride.  The  hydrogen  which  is  no  longer  combined 
escapes  (§  137).  When  iron  replaces  copper  in  copper 
sulphate,  a  new  compound  is  formed,  iron  sulphate  (§  157). 


(Hydrochloric 
acid) 

(Zinc 
chloride) 

(  Copper 
-I  Sulphur       = 
[Oxygen 
(Copper 
sulphate) 

(  Iron 
•<  Sulphur 
(  Oxygen 
(Iron 
sulphate) 

+     Copper 

Iron 


171.  The  Formation  of  New  Compounds  by  Double  Sub- 
stitution, or  Double  Decomposition. — In  order  to  illus- 
trate the  formation  of  compounds  by  direct  combination 
and  by  substitution,  simple  cases  have  been  selected* 
Usually  the  processes  are  more  complicated  but  the  prin- 
ciples remain  the  same.  One  of  the  most  interesting  and 
most  common  of  these  complicated  cases  is  seen  when 
two  compounds  are  brought  into  contact,  and  by  a  partial 
or  complete  interchange  of  elements  produce  new  com- 
pounds. 

When  two  compounds  are  brought  in  contact  with  each 
other,  one  of  the  elements  in  one  compound  may  be  sub- 
stituted for  one  of  the  elements  in  the  second  compound, 


CHEMISTRY,   ATOMS  181 

and  the  element  displaced  from  the  second  compound 
may  take  the  place  left  vacant  in  the  first  compound. 
This  evidently  results  in  an  interchange  of  elements 
between  the  compounds.  The  process  may  be  called 
double  substitution,  or,  if  emphasis  be  laid  on  the  breaking 
up  of  the  molecules  of  the  original  substance  preparatory 
to  the  formation  of  new  molecules,  it  may  also  be  called 
double  decomposition. 

Potassium  iodide  consists  of  potassium  and  iodine.  If 
solutions  of  lead  nitrate  and  potassium  iodide  are  brought 
together,  the  lead  and  potassium  exchange  places,  and  the 
new  compounds,  lead  iodide  and  potassium  nitrate,  result. 
The  lead  iodide  is  seen  as  a  bright  yellow  precipitate 
(§  159).  The  potassium  nitrate  remains  in  solution,  but 
if  the  water  is  filtered  and  then  boiled  the  potassium 
nitrate  remains  as  a  colorless  or  white  substance  resem- 
bling salt  in  appearance. 

IBS-  - 

(Lead  nitrate)  (Potassium  iodide)     (Lead  iodide)  (Potassium  nitrate) 

One  of  the  most  interesting  cases  of  double  decom- 
position is  the  formation  of  common  table  salt  and  water 
from  two  poisons,  sodium  hydrate  or  caustic  soda,  and 
hydrochloric  acid.  Dissolve  caustic  soda  in  water  (6 
parts,  by  weight,  of  caustic  soda  to  50  parts  of  water). 
Pour  half  a  test-tubeful  of  this  solution  into  a  porcelain 
evaporating  dish,  and  add,  drop  by  drop,  diluted  hydro- 
chloric acid  (4  parts  of  water  to  1  part  of  the  commercial 
acid).  Stir  the  mixture  after  the  addition  of  each  drop 
and  test  with  litmus  paper  (§  67),  until  the  mixture  no 
longer  turns  litmus  paper  from  red  to  blue.  By  this 
time  the  solution  is  likely  to  have  dissolved  considerable 


182  ELEMENTARY   PHYSICS 

litmus  from  the  litmus  paper.  If  the  number  of  drops 
of  acid  used  has  been  counted,  the  experiment  can  be 
brought  up  quickly  to  about  the  same  stage  by  taking 
another  half  test-tubeful  of  the  caustic  soda  solution, 
adding  at  once  the  number  of  drops  of  acid  used  before, 
and  then  testing  once  more  by  dipping  as  little  of  the 
litmus  paper  in  the  solution  as  possible.  If  too  much 
acid  has  been  used,  a  drop  or  more  of  caustic  soda  solu- 
tion must  be  added.  When  red  litmus  paper  is  no  longer 
changed  to  blue,  and  blue  litmus  paper  is  no  longer 
changed  to  red,  both  hydrochloric  acid  and  the  alkali, 
sodium  hydrate,  are  gone. 

Pour  a  few  drops  of  the  hydrochloric  acid  solution  into 
a  test-tube,  one-eighth  full  of  water.  Taste  one  drop  of 
the  mixture  on  a  glass  stirring  rod.  Taste  a  similarly 
diluted  solution  of  the  caustic  soda.  Taste  a  drop  of 
the  mixture  just  formed  in  the  evaporating  dish,  without 
diluting  the  mixture.  The  tastes  of  the  three  are  different : 
sour,  alkaline,  salty. 

Sodium  has  taken  the  place  of  hydrogen  in  hydro- 
chloric acid,  and  has  formed  common  table  salt.  At  the 
same  time  the  hydrogen  which  wras  displaced  has  taken 
the  place  vacated  by  the  sodium,  and  has  united  with  the 
oxygen  and  hydrogen,  and  formed  water.  The  solution 
left  in  the  evaporation  dish  is  simply  a  solution  of  salt 
in  water.  If  the  water  is  evaporated,  the  salt  is  left  be- 
hind. If  the  solution  has  not  dissolved  too  much  litmus, 
the  salt  left  in  the  evaporating  dish  is  pure  and  white. 


(  Sodium 
•<  Oxygen 
(  Hydrogen 

(  Hydrogen 
1  Chlorine 

j  Sodium 
1  Chlorine 

j  Hydrogen 
{  Oxygen 

(Sodium 
hydrate) 

(Hydrochloric 
acid) 

(Salt) 

(Water) 

GHEMISTEY,   ATOMS  183 

172,  The   Law  of  Conservation   of  Mass.— If  any  of  the 
preceding1  chemical  experiments  be  performed  in  a  vessel 
closed  in  such  a  manner  that  it  is  impossible  for  any 
material  to  enter  it  or  to  escape  from  it,  it  will  be  found 
that  the  total  weight  of  the  vessel  and  of  its  contents  is 
the  same  both  before  and  after  the  cheinical  change  takes 
place.     In  other  words,  there  is  neither  gain  nor  loss  of 
material  during  any  chemical  change.     The  change  con- 
sists merely  in  a  different  arrangement  of  the  elements. 
A  compound  may  be  separated  into  its  components  or  ele- 
ments.    Elements  may  unite  to  form  a  compound.     Or, 
the  components  of  different  compounds  may  first  separate 
and  may  then  unite  in  such  a  manner  as  to  form  com- 
pounds different  from  the  original  substances.     In  each 
case  the  same  fact  is  observed. 

The  total  weight  of  all  the  products  of  a  chemical  change 
is  exactly  equal  to  the  sum  of  all  the  weights  of  all  the  sub- 
stances affected  by  the  change. 

173.  Law  of   Definite    Proportions  by  Weight— It   has 
been  shown  that  the  red  oxide  of  mercury  consists  of 
mercury  and  oxygen.     It  is  possible  to   separate  these 
elements  in  such  a  manner  that  the  weight  of  each  can 
be  determined  accurately.     The  weight  of  mercury  and 
oxygen  present  in  red  oxide   of    mercury  depends,   of 
course,  upon  the   quantity  of  the  compound  taken  for 
analysis.     If,  however,  a  number  of  analyses  are  made 
and  the  weights  of  the  component  elements  are  deter- 
mined, the  interesting  fact  is  discovered  that  in  all  cases 
the  quantities  of  mercury  and  oxygen  are  present  in  the 
same  relative  proportion.     If,  for  instance,  28.35,  49.95, 
189,  and  245.83  ounces  of  the  red  oxide  of  mercury  were 
analyzed,  the  following  weights  of  mercury  and  oxygen 
would  be  obtained : 


184  ELEMENTARY  PHYSICS 

Oxide  of  Proportion  of  Mercury 

Mercury.  Mercury.  Oxygen.  to  Oxygen. 

28.35  oz.  =  26.25  +  2.10  12.5  :  1 

49.95  =  46.25  +  3.70  12.5  :  1 

189.00  =  175.00  +  14.00  12.5  :  1 

245.83  =  227.62  +  18.21  •     12.5  :  1 

If,  in  a  similar,  manner  different  quantities  of  water  are 
analyzed,  the  relative  proportion  of  oxygen  and  hydrogen 
present  is  found  to  be  always  the  same :  the  weight  of 
the  oxygen  is  in  each  case  8  times  the  weight  of  the 
hydrogen. 

The  constancy  of  the  relative  proportion  of  the  ele- 
ments in  different  quantities  of  the  same  compound  may 
also  be  shown  by  carefully  determining  the  weights  of 
the  different  elements  which  enter  into  combination  dur- 
ing the  formation  of  some  compound. 

If  8  times  as  great  a  weight  of  oxygen  as  of  hydro- 
gen is  imprisoned  in  any  long  test-tube  inverted  in  water, 
and  if  a  spark  of  electricity  is  sent  through  the  mixture 
(§  149),  all  of  the  mixture  enters  into  combination,  and 
forms  water.  Since  the  water  formed  from  the  gas  occu- 
pies much  less  volume  than  the  gas,  this  results  in  a  tem- 
porary vacuum  and  the  water  in  which  the  test-tube  is 
inverted  rushes  up,  so  that  the  water  formed  from  the 
mixture  of  oxygen  and  hydrogen  and  the  water  rushing 
up  into  the  test-tube  from  the  outside  completely  fill  the 
tube. 

If,  however,  other  proportions  are  taken,  all  of  the  gase- 
ous mixture  does  not  enter  into  combination.  If  1.72 
ounces  of  hydrogen  and  15.41  ounces  of  oxygen  are  placed 
in  a  vessel  and  united  by  sending  a  spark  of  electricity 
through  the  mixture  (Fig.  85  A),  and  if  the  water  is  al- 
lowed to  enter  the  vessel  co  as  to  fill  the  partial  vacuum, 
it  is  found  that  the  water  resulting  from  combination,  and 


CHEMISTRY,   ATOMS 


185 


the  water  rushing  in  from  the  outside  are  prevented  from 
filling  all  the  space  in  the  vessel  by  the  presence  of  some 
gas,  evidently  a  part  of  the  original  mixture.  If  the  gas 
left  uncombined  is  examined  it  is  found  that  the  gas  is 
pure  oxygen  and  that  its  weight  is  1.65  ounces  (Fig.  85  B). 
Therefore  all  the  hydrogen  entered  into  combination, 
but  only  13.76  ounces  of  oxygen  united  with  the  hydro- 
gen. But  13.76  ounces  are  exactly  8  times  1.72  ounces. 
Therefore  exactly  8  times 
as  much  oxygen  as  hydro- 
gen enters  into  combina- 
tion, even  if  an  excess  of 
oxygen  exists  in  the  mixt- 
ure. If  an  excess  of  hy- 
drogen had  been  used,  only 
J  as  much  hydrogen,  by 
weight,  as  oxygen  would 
have  entered  into  combina- 
tion. The  remainder  of  the 
hydrogen  would  have  re- 
mained uncombined. 

In  actual  experiments 
the  weights  of  the  mate- 
rials used  are  much  smaller 
than  those  indicated  by 
the  figures  here  given,  but 
the  principle  remains  the 
same. 

The  same  principle  may  be  readily  shown,  provided  a 
delicate  balance  is  at  hand.  Place  different  weights  of 
magnesium  (in  the  form  of  ribbon)  in  porcelain  crucibles, 
and  heat  the  crucibles  until  the  magnesium  unites  with 
the  oxygen  in  the  air,  and  produces  a  white  compound, 


FIG.  85. 


186  ELEMENTARY   PHYSICS 

magnesium  oxide,  easily  reduced  to  powder  by  a  mere 
touch.  The  quantity  of  oxygen  entering  into  combina- 
tion, as  shown  by  the  increased  weight  of  the  material  in 
the  crucible,  varies  in  each  case,  but  the  proportion  of 
oxygen  as  compared  with  that  of  the  magnesium  is  always 
the  same  :  in  each  case  the  weight  of  the  oxygen  entering 
into  combination  is  f  of  that  of  the  magnesium.  Of  all  the 
oxygen  in  the  air  only  that  quantity  enters  into  combi- 
nation which  is  necessary  to  preserve  this  proportion.  If 
not  enough  oxygen  were  at  hand,  a  part  of  the  magnesium 
ribbon  would  remain  uncombined. 

From  these  and  similar  facts,  the  following  law,  known 
as  the  Law  of  Definite  Proportions,  has  been  derived: 

The  same  chemical  compound  always  contains  the  same 
elements  in  the  same  proportion  l>y  weight. 

The  proportion  of  the  weight  of  any  element,  forming  part 
of  a  compound,  to  the  total  weight  of  the  compound  is  always 
the  same. 

174.  The  Law  of  Multiple  Proportions  by  Weight.-  When- 
ever hydrogen  and  oxygen  unite  directly,  they  unite  in 
one  proportion  only  and  always  form  water.  However,  by 
a  process  of  double  substitution  it  is  possible  to  secure 
another  compound  in  which  these  elements  are  united  in 
a  different  proportion.  When,  for  instance,  small  quanti- 
ties of  barium  dioxide  are  introduced  in  succession  into 
cold,  dilute  hydrochloric  acid,  the  barium  and  hydrogen 
exchange  places,  and  two  new  compounds  are  formed, 
barium  chloride  and  hydrogen  dioxide. 

(  Barium    _,      \  Hydrogen  j  Barium      ,      \  Hydrogen 

\  Oxygen    ~ "    \  Chlorine       "    {  Chlorine  ~"~    \  Oxygen 

Now  when  the  hydrogen  dioxide  is  analyzed  it  is  dis- 
covered that  the  proportion  of  oxygen  is  much  greater 


CHEMISTRY,   ATOMS 


187 


than  in  water ;  in  fact,  there  is  16  times  as  much  oxygen, 
by  weight,  as  hydrogen,  while  in  water  the  proportion  of 
oxygen  is  only  8  times  that  of  hydrogen.  This  shows 
that  two  elements  can  unite  in  more  than  one  proportion, 
in  order  to  form  compounds. 

The  hydrogen  dioxide  has  very  different  properties 
from  water.  It  is  a  syrupy  liquid,  and  weighs  1.45  times 
as  much  as  water.  It  corrodes  the  skin,  destroys  the  col- 
oring matter  in  plants,  and  is  used  in  surgical  operations 
to  kill  any  germs  which  may  come  in  contact  with  the 
wound.  It  readily  loses  half  of  its  oxygen  when  heated 
to  200°  F.,  and  the  remainder  becomes  ordinary  water. 
Owing  to  the  fact  that  hydrogen  dioxide  contains  exactly 
twice  as  much  oxygen  as  water,  the  relation  between  hy- 
drogen dioxide  and  water  may  be  expressed  in  the  follow- 
ing manner  (§  147) : 


(  Hydrogen 

-<  Oxygen  + 

(  Oxygen 

(Hydrogen  dioxide)    = 


(  Hydrogen 
I  Oxygen 

(Water) 


=  Oxygen 

+  (Oxygen) 


(Hydrogen  dioxide) 


(Water) 


(Oxygen) 


Hydrogen  dioxide,  usually  more  or  less  diluted  with 
water,  may  be  purchased  under  the  name  hydrogen  per- 
oxide. 

Cases  in  which  elements  unite  in  more  than  one  pro- 
portion are  rather  common,  but  different  conditions  must 
be  utilized  to  induce  the  elements  to  unite  in  these  differ- 


188  ELEMENTARY   PHYSIOS 

ent  proportions,  and  each  proportion  produces  a  different 
compound  with  different  properties. 

Nitrogen  and  oxygen  unite  in  5  different  proportions. 
When  these  elements  unite  in  such  proportions  that  the 
weight  of  the  oxygen  is  exactly  .57  as  great  as  that  of  the 
nitrogen,  nitrogen  monoxide  is  produced.  It  is  a  colorless 
and  odorless  gas,  but  possesses  a  sweetish  taste.  It  is 
used  by  dentists  to  deaden  pain,  and  is  often  called  laugh- 
ing gas. 

When  nitrogen  and  oxygen  unite  in  such  proportion  by 
weight,  that  the  oxygen  weighs  exactly  1.14  times  as 
much  as  the  nitrogen,  the  compound  nitrogen  dioxide  is 
produced.  It  is  a  colorless  gas,  and  is  chiefly  character- 
ized by  the  fact  that  as  soon  as  it  is  exposed  to  the  air 
it  takes  up  more  oxygen,  and  forms  red  vapors,  nitrogen 
tetroxide. 

When  the  proportion  by  weight  in  which  the  elements 
unite  is  1.71  times  as  much  oxygen  as  nitrogen,  the 
compound  nitrogen  trioxide  is  produced.  It  is  a  blue 
liquid. 

When  2.28  times  as  much  oxygen  as  nitrogen  unite, 
the  compound  nitrogen  tetroxide  is  produced.  It  is  an 
orange-brown  liquid.  Its  vapor,  however,  is  red,  and  is 
very  corrosive  and  dangerous  to  inhale. 

When  2.85  times  as  much  oxygen  as  nitrogen  unite, 
nitrogen  pentoxide  is  produced.  It  is  a  white  solid  sub- 
stance and  is  liable  to  explode. 

An  examination  of  the  last  four  compounds  shows  that 
the  proportion  of  oxygen  as  compared  with  the  nitrogen 
is  2,  3,  4,  and  5  times  the  proportion  of  oxygen  present  in 
the  first  compound,  nitrogen  monoxide. 

The  chemical  composition  of  these  compounds  may  be 
expressed  in  the  following  manner  : — 


CHEMISTRY,   ATOMS  189 


j  Nitrogen 
I  Oxygen 

(Nitrogen 
monoxide) 

fNitrogen 
j  Oxygen 
lOxygen 

(Nitrogen 
dioxide) 

fNitrogen 
I  Oxygen 
I  Oxygen 
lOxygen 

(Nitrogen 
trioxide) 

rNitrogen 
Oxygen 
-!  Oxygen 
Oxygen 
VOxygen 

(Nitrogen 
tetroxide) 

1  Nitrogen 
Oxygen 
Oxygen 
Oxygen 
Oxygen 
Oxygen 
(Nitrogen 
pentoxide) 

In  many  cases  the  series  are  not  so  complete.  For  in- 
stance, iron  and  phosphorus  unite  in  the  following  pro- 
portions with  chlorine: 

{Iron  flron  ^Phosphorus         ^Phosphorus 

Chlorine       J  Chlorine       J  Chlorine  I  Chlorine 

Chlorine        j  Chlorine         j  Chlorine  J  Chlorine 

I  Chlorine         I  Chlorine  1  Chlorine 

Chlorine 
l.Chlorine 

If  all  the  substances  be  studied  in  which  the  same 
elements  unite  in  more  than  one  proportion,  it  will  be 
found  that  the  numbers  expressing  the  various  combining 
weights  of  one  element  with  a  given  quantity  of  another 
element  are  all  multiples  of  some  one  number.  These 
facts  can  be  expressed  in  the  form  of  the  following  law, 
known  as  the  Law  of  Multiple  Proportions. 

When  different  weights  of  one  element  enter  into  combination 
with  a  fixed  weight  of  another  element ,  the  different  weights 
of  the  first  element  form  simple  ratios  with  each  other. 

175.  Elements  Often  Exist  in  Compounds  as  Equal,  Dis- 
tinct, Separate  Quantities.— In  the  preceding  paragraphs 
it  has  been  shown  that  the  proportion,  by  weight,  of  oxy- 
gen and  hydrogen  is  exactly  twice  as  great  in  hydrogen 
dioxide  as  in  water:  moreover,  that  the  oxygen  acts  as 
though  half  of  it  were  not  as  well  united  to  the  hydrogen 
in  hydrogen  dioxide  as  the  other  half  of  the  oxygen,  since, 
on  heating  the  hydrogen  dioxide,  half  of  the  oxygen 
readily  leaves  the  compound,  and  thus  changes  hydro- 


190  ELEMENTARY   PHYSICS 

gen  dioxide  into  water.  The  other  half  of  the  oxygen 
clings  more  tightly  to  the  hydrogen,  and  it  therefore  re- 
quires a  much  higher  temperature  to  dissociate  it  from 
the  hydrogen  in  water.  Even  at  ordinary  temperatures 
hydrogen  dioxide  slowly  loses  its  oxygen,  but  above 
200  degrees  F.  half  of  the  oxygen  passes  off  so  readily 
that  the  liquid  seems  to  boil  briskly.  In  order  actually 
to  separate  the  oxygen  from  the  hydrogen  in  water  by 
means  of  heat,  a  much  higher  temperature,  1230  degrees 
to  1280  degrees  Fahrenheit,  must  be  applied  to  water 
after  it  has  been  changed  to  the  form  of  steam. 

Since  oxygen  appears  to  exist  in  hydrogen  dioxide  in 
two  quantities,  not  equally  well  united  to  the  hydrogen, 
the  question  arises,  may  the  oxygen  in  water  be  also  sepa- 
rated into  two  or  more  distinct  quantities  so  that  part 
may  be  removed  while  the  other  remains  in  combination? 
No  method  for  accomplishing  this  is  known  at  present 
and  chemists  consider  it  impossible.  Whenever  any 
quantity  of  oxygen  is  set  free,  the  proportionate  quantity 
of  hydrogen  is  also  set  free.  The  result  is  the  complete 
breaking  up  of  the  water  affected  into  its  component  ele- 
ments. Only  the  water  which  has  not  yet  lost  its  oxygen 
remains  behind  in  the  vessel. 

However,  a  method  is  known  of  separating  a  part  of 
the  hydrogen  from  water,  leaving  the  rest  in  combination 
with  the  oxygen.  In  this  case  the  part  of  the  water  af- 
fected loses  exactly  half  of  its  hydrogen.  In  order  to 
cause  this  separation  heat  is  not  sufficient.  A  chemical 
change  is  necessary.  Some  other  element  must  take  the 
place  of  that  half  of  the  hydrogen  which  was  separated. 
One  of  the  simplest  means  of  accomplishing  this  is  by 
placing  sodium  in  a  vessel  containing  a  small  quantity  of 
water.  Sodium  takes  the  place  of  part  of  the  hydrogen, 


O  vjf    I     f 


CHEMISTRY,    ATOMS 


191 


forming-  a  new  compound,  sodium  hydrate,  while  the  hy- 
drogen displaced  escapes.  If  sodium  is  added  until  all 
of  the  water  has  been  changed  to  sodium  hydrate,  or  if, 
after  some  sodium  has  been  added,  the  water  not  affected 
is  removed  by  boiling,  the  new  compound  becomes  visi- 
ble. It  is  a  white  crystalline  solid,  and  is  usually  sold 
under  the  name  caustic  soda.  If  this  chemical  is  analyzed 
it  is  found  that  the  proportion  of  hydrogen  compared 
with  oxygen  is  just  half  as  great  in  sodium  hydrate  as  in , 
water.  The  other  half  of  the  hydrogen  in  water  has  been 
replaced  by  sodium. 

The  removal  of  the  second  half  of  the  hydrogen  which 
was  originally  a  part  of  the  water,  but  which  is  now  a 
part  of  the  sodium  hydrate,  requires  the  application  of 
heat.  In  this  case  40  parts  by  weight  of  solid  sodium 
hydrate  should  be  heated  with  23  parts  of  sodium.  The 
hydrogen  remaining  as  part  of  the  sodium  hydrate  after 
the  preceding  experiment,  then  is  also  displaced  by 
sodium,  and  the  compound  sodium  oxide  is  formed. 

These  chemical  changes  may  be  represented  as  follows, 
designating  the  word  sodium  by  the  letters  Na  : 


H 


(Water) 


(Sodium)  (Sodium  hydrate)          (Hydrogen) 


Na 


0 


(Sodium  hydrate)  (Sodium)  (Sodium  oxide)  (Hydrogen) 


192 


ELEMENTARY  PHYSICS 


If  iron  pyrites  is  heated  for  a  long  time  at  a  high  tem- 
perature in  a  covered  crucible,  half  of  the  sulphur  is 
driven  off  and  the  compound  iron  sulphide  remains.  This 
may  be  represented  diagrammatically  as  follows,  the  word 
iron  being  represented  by  Fe : 


(Iron  pyrites)          (Iron  sulphide)        (Sulphur) 

As  the  result  of  many  similar  experiments  it  has  been 
learned  that  in  many  compounds  one  or  more  of  the  ele- 
ments seem  to  exist  as  though  in  equal,  distinct,  and 
separable  quantities ;  as  though  in  water  there  were  two 
equal,  separable  quantities  of  hydrogen  and  one  insepa- 
rable quantity  of  oxygen,  and  as  though  in  hydrogen  di- 
oxide there  were  two  equal,  separable  quantities  of  hy- 
drogen and  two  equal,  separable  quantities  of  oxygen. 

176.  The  Molecules  of  Compounds  Must  Have  the  Same 
Chemical  Composition  as  the  Compounds  Themselves. — Our 
conception  of  molecules  demands  that  molecules  of  the 
same  substance  be  considered  exactly  alike  in  every  re- 
spect. The  fact  that  crystals  of  the  same  substance 
have  the  same  form,  indicates  that  the  molecules  also  of 
these  substances  are  alike  in  size  and  form  (§§  86,  87). 
Since  the  same  substances  always  consist  of  the  same  ele- 
ments, combined  in  the  same  proportion  (§  173),  their 
molecules  must  all  be  alike  in  chemical  composition. 

If  the  red  oxide  of  mercury  consists  of  mercury  and  oxy- 
gen in  the  proportion,  by  weight,  of  12.5  to  1  (§  173), 
then  each  single  molecule  of  this  substance  also  consists 
of  mercury  and  oxygen  united  in  this  same  proportion, 


CHEMISTRY,   ATOMS  193 

Moreover,  whenever  any  element  in  any  compound  seems 
to  act  as  if  it  consisted  of  several  equal,  distinct,  and  sepa- 
rable quantities  (§  175),  it  may  be  assumed  that  in  each 
molecule  of  the  compound  this  element  consists  of  the 
same  number  of  equal,  separable  quantities. 

For  example,  it  may  be  assumed  that  in  the  case  of  a 
molecule  of  water,  there  are  two  equal,  distinct,  and  sepa- 
rable quantities  of  hydrogen  but  only  one  distinct  quan- 
tity of  oxygen. 

Finally,  since  these  equal  quantities  of  the  element  en- 
tering into  combination  seem  to  be  indivisible  into  still 
smaller  quantities,  it  may  be  assumed  that  in  the  case  of 
each  molecule  also  the  equal,  distinct,  separable  quanti- 
ties of  this  element  are  not  further  divisible. 

In  the  case  of  water,  this  theory  assumes  that  each 
molecule  consists  of  two  equal,  distinct,  separable  quan- 
tities of  hydrogen,  which  are  not  further  divisible  into 
smaller  portions,  but  that  there  is  only  one  such  quantity 
of  oxygen. 

177.  Atoms. — To  the  smallest,  indivisible  quantity  of  an 
element  present  in  any  molecule  the  name  atom  has  been 
given.  A  molecule  of  water  is  therefore  supposed  to  con- 
sist of  2  atoms  of  hydrogen  and  1  atom  of  oxygen,  while  a 
molecule  of  hydrogen  dioxide  consists  of  2  atoms  of  hy- 
drogen and  2  atoms  of  oxygen  : 

oo  Hydrogen 
(J  Oxygen 

Every  element  is  believed  to  have  its  own  kind  of  atom. 
The  atoms  of  the  same  element  are  all  alike,  having  the 
same  size,  shape  and  weight,  but  the  atoms  of  different 
elements  are  unlike.  Thus  every  atom  of  oxygen  is  like 
every  other  atom  of  oxygen,  but  not  like  the  atoms  of  any 


194  ELEMENTARY   PHYSICS 

other  element.     There  are  then  as  many  different  kinds 
of  atoms  as  there  are  elements. 

Atoms  have  an  attraction  for  each  other,  which  causes 
them  to  unite  and  to  form  molecules.  This  force  of  attrac- 
tion which  brings  the  atoms  together  is  called  chemical 
affinity.  Molecules  may  be  considered  as  consisting  of 
groups  of  atoms  held  together  by  chemical  affinity. 

178.  Chemical  Changes  Involving  Large  Quantities  of  a 
Substance  may  be  Discussed  as  if  Affecting  only  One  Mole- 
cule.— Since  a  molecule  is  nothing  more  nor  less  than  the 
smallest  particle  of  any  substance  which  can  exist,  it  is 
evident  that  no  chemical  change  can  take  place  in  a  body 
unless  the  chemical  character  of  the  individual  molecules 
is  altered.     If  the  experiment  is  conducted  in  such  a 
manner  that  only  a  part  of  the  molecules  of  the  com- 
pound are  affected,  then,  at  the  close  of  the  chemical  opera- 
tion, two  compounds  will  be  present :  the  unchanged  por- 
tion of  the  original  compound,  and  as  much  of  the  new 
compound  as  resulted  from  the  chemical  alteration  of  that 
part  of  the  original  substance  which  was  actually  affected. 
For  the  sake  of  simplicity,  in  order  to  make  clear  the 
general  principles  underlying  chemical   changes,   it  is 
always  assumed  that  in  each  case  under  discussion  the 
process  involved  has  been  carried  far  enough  to  affect 
every  molecule.      This    being    the   case,   the    chemical 
changes  affecting  the  entire  mass  of  the  substance  acted 
upon    are   represented    in   miniature    by    the    chemical 
changes  taking  place  in  any  one  of  the  molecules.     In 
discussing  chemical  changes  it  is  therefore  usually  con- 
venient to  discuss  them  as  if  affecting  only  one  molecule 
of  each  substance  acted  upon. 

179,  Multiple    Proportions    Explained    by    Existence   of 
Atoms. — The  theory  that  elements  are  not  more  or  less 


CHEMISTRY,   ATOMS  195 

continuous  substances  but  are  made  up  of  separate  atoms 
will  explain  a  number  of  facts  otherwise  inexplainable. 
It  may  be  shown  by  chemical  analysis  that  the  same  com- 
pound always  consists  of  the  same  elements  united  in  the 
same  proportion.  Water,  for  instance,  consists  of  hydro- 
gen and  oxygen  united  in  the  proportion  of  2  parts  of 
hydrogen  to  16  parts  of  oxygen.  According  to  the  atomic 
theory  each  molecule  of  water  consists  of  two  atoms  of 
hydrogen  and  one  atom  of  oxygen,  the  oxygen  atom 
weighing  16  times  as  much  as  one  of  the  hydrogen 
atoms.  Since  each  atom  of  oxygen  is  like  every  other 
atom  of  oxygen,  and  since  each  atom  of  hydrogen  is  like 
every  other  atom  of  hydrogen,  the  relative  amount  of 
oxygen  and  hydrogen  in  each  molecule  of  water  must 
always  be  the  same. 

Oxygen  may  unite  with  hydrogen  in  another  propor- 
tion, differing  from  that  present  in  water.  It  is  evident, 
however,  that  if  the  second  compound  differs  from  the 
first  compound  only  in  the  relative  amount  of  oxygen 
present,  each  molecule  of  the  second  compound  must 
contain  more  than  one  atom  of  oxygen.  It  cannot  con- 
tain a  fractional  quantity  of  oxygen,  since  atoms  are  in- 
divisible. This  second  compound  is  hydrogen  dioxide. 
Its  molecules  consist  of  2  atoms  of  hydrogen  and  2  atoms 
of  oxygen.  Whenever  one  of  the  atoms  of  oxygen  is 
driven  off  by  heat,  a  molecule  of  water  is  left  behind. 

Oxygen  unites  in  more  than  one  proportion  also  with 
many  other  elements.  It  unites  in  5  proportions  with 
the  same  weight  of  nitrogen  producing  5  different  com- 
pounds. Since  a  molecule  represents  in  miniature  what 
is  true  of  the  entire  mass  of  a  compound,  the  number  of 
atoms  of  nitrogen  may  be  considered  the  same  in  every 
molecule  of  all  five  compounds.  In  this  case,  however, 


196  ELEMENTARY   PHYSICS 

the  number  of  atoms  of  oxygen  present  in  the  molecules 
of  the  different  compounds  cannot  be  the  same.  The 
conception  of  atoms  of  the  same  substance  as  equal  and 
indivisible  excludes  all  possibility  of  the  extrance  of  an 
element  into  combination  except  in  the  form  of  entire 
atoms.  Therefore  the  number  of  atoms  of  oxygen  pres- 
ent in  the  molecules  of  each  o^  these  five  compounds 
must  be  1,  2,  3,  4,  or  some  other  whole  number  of  atoms. 

According  to  chemists  the  number  of  atoms  of  nitrogen 
present  in  the  molecules  of  all  of  these  compounds  is  2. 
With  these  two  atoms  of  nitrogen  are  united  1,  2,  3,  4, 
and  5  atoms  of  oxygen.  Therefore,  the  relative  weight 
of  oxygen  united  with  nitrogen  in  these  five  compounds 
must  be  1,  2,  3,  4,  and  5  times  the  relative  weight  shown 
by  that  compound  whose  molecules  contain  only  one 
atom  of  oxygen. 

180.  The  Weights  of  the  Atoms  of  Elements  Compared 
with  the  Weight  of  an  Atom  of  Hydrogen. — It  is  impossible 
to  determine  the  actual  weight  of  an  atom,  but  if  our  ideas 
as  to  the  number  of  atoms  in  various  compounds  are 
correct,  it  is  possible  at  least  to  determine  their  relative 
weights ;  in  other  words,  to  determine  how  much  greater 
the  weight  of  one  kind  of  atom  is  than  the  weight  of 
another  kind  of  atom. 

In  determining  the  relative  weights,  or  what  are  be- 
lieved to  be  the  relative  weights,  of  atoms,  the  weight  of 
one  atom  of  hydrogen,  which  is  considered  the  lightest 
atom,  has  been  taken  as  a  standard.  The  weight  of  an 
atom  of  hydrogen  is,  therefore,  called  1,  and  the  weights 
of  all  other  kinds  of  atoms  are  compared  with  this  unit 
(§  182). 

It  is  known  that  in  the  formation  of  hydrochloric  acid 
1  part,  by  weight,  of  hydrogen  unites  with  35.2  parts,  by 


CHEMISTEY,   ATOMS  197 

weight,  of  chlorine.  If  the  theory  accepted  by  chemists 
be  true  that  a  molecule  of  this  acid  consists  of  1  atom  of 
hydrogen  and  1  atom  of  chlorine,  then,  if  the  weight  of 
one  atom  of  hydrogen  be  called  1,  the  weight  of  1  atom  of 
chlorine  must  be  35.2. 

In  the  formation  of  water  1  part,  by  weight,  of  hydro- 
gen unites  with  8  parts,  by  weight,  of  oxygen.  If  it  be 
true  that  water  consists  of  2.  atoms  of  hydrogen  and  1 
atom  of  oxygen,  then  1  atom  of  oxygen  weighs  8  times  as 
much  as  2  atoms  of  hydrogen,  or  16  times  as  much  as  1 
atom  of  hydrogen. 

In  ammonia  gas  4f  parts  by  weight  of  nitrogen  unite 
with  1  part  by  weight  of  hydrogen.  If  the  molecule  of 
ammonia  consists  of  1  atom  of  nitrogen  and  3  atoms  of 
hydrogen,  then  1  atom  of  nitrogen  weighs  4f  times  as 
much  as  3  atoms  of  hydrogen,  or  14  times  as  much  as  1 
atom  of  hydrogen. 

This  discussion  will  be  sufficient  to  show  that  if  our 
ideas  about  the  number  of  atoms  in  the  molecules  of 
various  compounds  are  correct,  it  is  possible  to  compare 
the  weights  of  the  atoms  united  in  the  same  compound, 
and,  by  a  series  of  comparisons,  to  compare  the  weight  of 
any  atom  with  the  weight  of  the  atom  of  hydrogen.  The 
determination  of  the  number  of  atoms  in  a  compound  is 
a  matter  of  theory.  The  reasoning  involved  is  often  too 
difficult  for  an  elementary  text-book.  Enough  has  been 
said  in  preceding  paragraphs  to  give  some  notions  on  this 
subject  (§§  174—179). 

181.  Names  of  Elements  Represented  by  Symbols. — In  the 
discussion  of  chemical  phenomena  it  is  often  convenient 
to  be  able  to  represent  the  names  of  elements  by  one  or 
two  letters.  Some  names  are  represented  by  the  initial 
letter,  capitalized  always.  Carbon  is  represented  by  C, 


198 


ELEMENTARY   PHYSICS 


hydrogen  by  H,  nitrogen  by  N,  oxygen  by  O,  phosphorus 
by  P,  sulphur  by  S.  When  several  names  begin  with  the 
same  letter,  two  letters  are  used,  the  initial  letter  together 
with  a  letter  occurring  later  in  the  word.  The  first  letter 
is  capitalized  and  the  second  is  not.  Aluminum  is  repre- 
sented by  Al,  arsenic  by  As,  chlorine  by  Cl,  manganese 
by  Mn,  nickel  by  Ni,  platinum  by  Pt,  zinc  by  Zn.  In 
selecting  these  letters  the  Latin  names  of  the  elements 
are  used.  For  that  reason  the  letters  used  to  represent 
the  names  of  some  elements  will  not  be  recognized  at 
once  by  those  not  familiar  with  the  Latin  names.  For 
example,  antimony  (stibium  in  Latin)  is  represented  by 
Sb,  copper  (cuprum)  by  Cu,  gold  (aurum)  by  Au,  iron 
(ferrum)  by  Fe,  lead  (plumbum)  by  Pb,  mercury  (hydrar- 
gyrum) by  Hg,  potassium  [kaliumj  by  K,  silver  [argen- 
tum]  by  Ag,  sodium  [natrium]  by  Na,  and  tin  [stannum] 
by  Sn.  The  letters  are  called  symbols  of  the  elements. 

182.  Table  of  Elements,  Including  Their  Symbols  and 
Atomic  Weights. — The  following  is  a  list  of  the  more 
common  elements.  The  names  of  the  metals  [§  187]  are 
in  italics.  The  non-metals  [§  187]  are  printed  in  ordi- 
nary type.  The  symbols  are  given  in  the  second  and  the 
atomic  weight  in  the  third  column.  A  few  substances 
act  both  as  metals  and  as  non-metals. 


Name 

Symbol 

Atomic  Wt. 

Name 

Symbol 

Atomic  Wt. 

Aluminum 

Al 

27.0 

Chlorine 

Cl 

35.2 

Antimony 

Sb 

119.0 

Chromium 

Cr 

51.7 

Arsenic 

As 

74.4 

Cobalt 

Co 

58.6 

Barium 

Ba 

136.4 

Copper 

Cu 

63.0 

Bismuth  ? 

Bi 

206.5 

Fluorine 

F 

19.0 

Bromine 

Br 

79.4 

Gold 

Au 

196.0 

Cadmium 

Cd 

111.4 

Hydrogen 

H 

1.0 

Calcium 

Ca 

40.0 

Iodine 

I 

126.0 

Carbon 

C 

12.0 

Iridium 

Ir 

192.7 

CHEMISTRY,   ATOMS 


199 


Name 

Symbol 

Atomic  Wt.           Name 

Symbol 

Iron 

Fe 

55.6 

Platinum 

Pt 

Lead 

Pb 

205.4 

Potassium 

K 

Lithium 

Li 

7.0 

Silicon 

Si 

Magnesium 

Mg 

24.0 

Silver 

Ag 

Manganese. 

Mn 

55.0 

Sodium 

Na 

Mercury 

Hg 

198.5 

Strontium 

Sr 

Nickel 

Ni 

58.2 

Sulphur 

S 

Nitrogen 

N 

14.0 

Tin 

Sn 

Oxygen 

O 

15.8 

Zinc 

Zn 

Phosphorus 

P 

30.8 

Atomic  Wt. 

193.6 
39.0 
28.2. 

107.1 
23.0 
87.0 
32.0 

118.0 
65.0 


183.  The  Formula  of  a  Compound  Records  the  Number 
of  Atoms  of  each  Element  Supposed  to  be  Present  in  each 
Molecule. — The  ability  to  represent  a  long-  name  by  means 
of  a  few  letters  is  of  great  convenience,  not  only  when  it 
is  necessary  to  use  the  names  of  elements,  but  also  when 
it  is  desirable  to  use  names  of  chemical  compounds.  In 
the  system  in  actual  use  no  attempt  is  made  to  abbreviate 
the  names  of  compounds,  but  a  combination  of  letters  is 
chosen  which  will  indicate,  at  a  glance,  what  is  believed 
to  be  the  chemical  composition  of  each  molecule  of  the 
compound.  This  combination  of  letters  is  called  the 
formula  of  the  molecule.  In  these  formulas  each  symbol 
represents  a  single  atom  of  one  of  the  component  ele- 
ments. For  instance,  red  oxide  of  mercury  is  represented 
by  the  formula  HgO,  which  indicates  that  every  mole- 
cule of  the  red  oxide  of  mercury  is  believed  to  contain 
one  atom  of  mercury  and  one  atom  of  oxygen. 

When  more  than  1  atom  of  the  element  are  present,  a 
number,  which  indicates  how  many  atoms  of  the  element 
are  present  in  the  molecule,  is  written  at  the  lower  right- 
hand  corner  of  the  symbol  representing  the  element.  The 
formula  of  water,  H2O,  indicates  that  2  atoms  of  hydrogen 
and  1  of  oxygen  are  present  in  each  molecule  of  water. 


200  ELEMENTARY   PHYSICS 

In  the  same  manner,  a  single  molecule  of  potassium 
chlorate  may  be  represented  by  the  formula  KC1O3. 
This  indicates  that  each  molecule  of  the  substance  con- 
tains 1  atom  of  potassium,  1  atom  of  chlorine,  and  3  atoms 
of  oxygen. 

A  group  of  letters  enclosed  in  brackets,  with  a  small 
number  after  the  second  bracket,  indicates  that  the  en- 
tire group  of  atoms  represented  by  the  letters  enclosed 
in  the  brackets  must  be  multiplied  by  this  number  in 
order  to  ascertain  the  number  of  atoms  of  each  element 
in  the  molecule  of  the  compound.  Thus  Pb  (NO3)2  ,  lead 
nitrate,  consists  of  1  atom  of  lead,  2  of  nitrogen,  and  6  of 
oxygen.  In  this  case  the  group  of  atoms  in  that  part  of 
the  formula  which  is  enclosed  in  brackets  is  believed  to 
be  more  strongly  united  than  the  other  atoms,  so  that  in 
cases  of  substitution  it  often  enters  and  leaves  the  mole- 
cule as  a  group.  At  least  the  atoms  included  in  the  group 
often  enter  the  molecule  from  the  same  source.  Thus  the 
NO3  of  lead  nitrate  may  have  been  derived  from  nitric  acid, 
the  hydrogen  of  the  acid  having  been  displaced  by  lead. 
When  it  is  desirable  to  express  that  several  molecules 
are  present,  a  large  figure  representing  the  number  of 
molecules  is  placed  before  the  formula.  Thus,  3H2O  rep- 
resents 3  molecules  of  water. 

184.  How  to  Read  a  Formula. — A  glance  at  a  formula 
will  at  once  suggest  to  the  expert  chemist  the  name  of 
the  compound  ;  moreover,  it  will  also  suggest,  even  to  one 
not  an  expert  chemist,  the  chemical  composition  of  the 
compound.  In  speaking  of,  or  in  writing  about  a  sub- 
stance, it  is  often  more  convenient  to  give  the  formula  of 
its  molecule  than  to  use  its  actual  name.  In  this  case 
each  figure  and  letter  is  read  in  the  order  in  which  it 
occurs  in  the  formula.  A  slight  stop  is  made  after  every 


CHEMISTRY,   ATOMS  201 

figure  indicating  the  number  of  atoms.  Any  figure  given 
before  any  of  the  letters  of  the  formula  have  been  men- 
tioned is  at  once  understood  as  giving  the  number  of 
molecules  present.  The  reading  of  3H2SO4  is  three — H 
two — S — O  four,  and  4K2Cr2O7  is  read  four — K  two — Cr 
two — O  seven. 

Head  the  formulas,  P2O5,  CO2,  KC1O3,  Ca  CO3,  ZnSO4. 
In  every  case  tell  what  elements  and  how  many  of  their 
atoms  are  present  in  each  molecule. 

185.  The  Formula  of  a  Compound  Indirectly  Records 
the  Relative  Combining  Weights  of  the  Component  Ele- 
ments.— The  existence  of  both  molecules  and  atoms  is 
assumed  in  order  to  explain  phenomena,  often  very 
familiar  phenomena,  which  otherwise  are  inexplainable. 
Since  it  is  not  at  all  likely  that  molecules  and  atoms  will 
ever  be  seen,  our  ideas  of  them  and  of  their  properties 
must  necessarily  vary  with  the  increase  of  facts  which 
we  try  to  explain  when  we  assume  the  existence  of  mole- 
cules and  atoms.  As  our  knowledge  of  chemistry  increases 
it  is  fair  to  assume  that  new  methods  of  experimentation 
may  prove  that  elements  which  we  now  consider  to  be 
present  in  molecules  in  the  form  of  single  atoms,  may  in 
reality  be  represented  by  2  or  3  atoms,  or  that  elements 
now  believed  to  be  present  in  the  form  of  2  atoms  in  each 
molecule  may  later,  on  account  of  further  discoveries,  be 
believed  to  be  present  in  the  form  of  4  or  6  atoms  in  each 
molecule,  so  that  it  will  be  necessary  from  time  to  time 
to  change  some  of  the  formulas  in  order  to  represent  the 
new  facts. 

However,  while  our  conceptions  of  the  number  of  atoms 
of  each  element  in  the  molecules  of  various  compounds 
may  change,  there  is  one  fact  which  can  be  determined 
with  considerable  exactness,  regarding  which  there  are 


202  ELEMENTARY   PHYSICS 

likely  to  be  few  changes  of  opinion  in  the  future,  and  that 
is  the  relative  quantity  of  each  element  present  in  a  com- 
pound. If  the  element  has  been  accurately  identified,  if 
the  balances  are  good,  and  if  the  chemical  changes  taking 
place  during  all  the  processes  of  synthesis  and  analysis 
have  been  correctly  followed,  there  need  be  little  change  of 
opinion  as  to  the  relative  weights  of  the  elements  present. 
In  the  effort  to  express  the  atomic  constitution  of  each 
molecule  of  the  compound,  the  relative  quantity  of  each 
element  present  in  the  compound  is  also  expressed,  al- 
though only  indirectly.  For  instance,  if  the  formula  of 
nitric  acid  is  written  as  HNO3,  this  shows  that,  at  pres- 
ent, methods  of  chemical  analysis  suggest  that  1  atom  of 
hydrogen,  1  of  nitrogen  and  3  of  oxygen  are  present  in 
the  compound.  Since  at  present  the  weight  of  an  atom 
of  nitrogen  is  assumed  to  be  14  times  as  great  as  the 
weight  of  an  atom  of  hydrogen,  and  that  of  an  atom  of 
oxygen  to  be  16  times  as  great  as  that  of  an  atom  of  hy- 
drogen, it  is  evident  that  the  relative  proportions  of  hy- 
drogen, nitrogen,  and  oxygen  present  in  nitric  acid  must 
be  1:14:48.  This  relative  proportion  of  the  three  ele- 
ments in  nitric  acid  is  known  with  certainty.  It  has  been 
determined  by  chemical  analysis. 
Fact  But  the  existence  of  three  atoms 

n     I I     I I          of  oxygen,  with  only  one  atom  of 

^  ;    ^     :      Q  hydrogen     and     nitrogen     in    this 

compound,  is  merely  an  assumption 
(Fig.  86),  based,  however,  upon  facts 
o     C^\       (^Theory   which   are  just  as  well  established 
N         ^-.  as  those  discussed  in  the  previous 

V_y  paragraphs  in  connection  with  the 

°3  constitution  of  water  and  of 

FIG.  86.  T      .  -. 

gen  dioxide. 


CHEMISTRY,    ATOMS  203 

From  the  relative  proportions  of  the  elements  present 
in  a  compound,  as  established  by  actual  analysis,  and  in- 
directly recorded  in  the  formula  for  a  molecule  of  a  com- 
pound, it  is  possible  to  determine  what  weight  of  any 
element  is  present  in  any  compound.  If,  for  instance,  in 
the  case  of  nitric  acid,  the  elements  hydrogen,  nitrogen, 
and  oxygen  are  present  in  the  proportions  1 : 14 :  48,  then 
-fe  of  any  quantity  of  nitric  acid  must  be  hydrogen,  jf 
must  be  nitrogen,  and  f f  must  be  oxygen.  In  16  ounces 
of  nitric  acid,  ^  of  16  —  .254  ounce  of  the  acid  must  con- 
sist of  hydrogen,  £f  of  16  =  3.556  ounces  of  the  acid  must 
consist  of  nitrogen,  and  f  f  of  16  =  12.190  ounces  of  the 
acid  must  consist  of  oxygen. 

In  fact,  before  a  single  formula  is  ever  written,  some 
chemist  analyzes  the  compound  and  determines  the  rela- 
tive proportion  by  weight  of  all  the  elements  which  it 
contains.  These  relative  proportions  of  the  elements  he 
records  in  his  formula,  but,  instead  of  writing  his  formula 
in  the  form  of  .254H,  3.556N,  190O,  or  1H,  14N,  48O,  he 
attempts  to  express  the  results  of  his  analysis  in  such  a 
manner  that  their  application  may  be  general.  He  there- 
fore determines  from  his  chemical  analysis  how  many 
atoms  of  each  element  are  present  in  each  molecule  and 
records  the  number  of  atoms  in  the  formula.  Then  the 
number  of  atoms  of  the  various  elements  as  recorded  in 
the  formula  indicates  practically  also  the  relative  pro- 
portion by  weight  of  the  elements  present,  to  any  one  who 
knows  the  relative  weights  of  the  different  atoms. 

186.  Chemical  Changes  May  Be  Recorded  in  the  Form 
of  an  Equation. — The  amount  of  each  element  present  in 
any  particular  quantity  of  any  substance  may  be  deter- 
mined by  chemical  analysis.  Since  not  a  particle  of  sub- 
stance is  either  created  or  destroyed  during  any  chemical 


204  ELEMENTARY   PHYSICS 

change,  it  is  possible  to  express  in  the  form  of  an  equation 
the  relative  amount  of  each  element  present  in  each  sub- 
stance at  the  beginning-  of  the  experiment  and  in  what 
manner  these  elements  are  combined  in  the  different  prod- 
ucts resulting  from  the  chemical  change. 

In  practice,  no  attempt  is  made  to  express  in  equations 
the  actual  weight  of  the  different  materials  taking  part, 
but  only  their  relative  weights.  This  may  be  accom- 
plished by  representing  the  chemical  change  as  though  it 
were  taking  place  between  only  1,  2,  or  some  other  small 
number  of  molecules  of  the  substances  present.  The 
manner  in  which  the  atoms  are  united  before  the  chem- 
ical change  is  indicated  before  the  equality  sign.  The 
manner  in  which  the  atoms  are  united  after  the  chemical 
change  is  indicated  after  the  sign  of  equality.  In  other 
words,  the  substances  used  are  indicated  on  the  left  of 
the  sign  of  equality,  and  the  substances  resulting  from 
the  chemical  action  are  indicated  on  the  right  of  this  sign. 

To  illustrate,  the  chemical  action  which  took  place 
when  the  potassium  chlorate  was  heated  together  with 
the  manganese  dioxide  (§  134),  may  be  represented  by 

KC1O3  +  MnO2  =  KC1  +  3O  +  MnO2, 

which  means  that  after  the  chemical  change  took  place 
the  manganese  dioxide  was  found  unchanged,  but  the 
potassium  chlorate  was  found  broken  up  into  potassium 
chloride,  KC1,  and  oxygen. 

The  chemical  action  when  phosphorus  is  burned  in  air 
may  be  represented  by  the  equation, 

2P  +  5O  +  20N  =  P2O5  +  20N, 

which  means  that,  after  the  chemical  change  has  taken 
place,  the  nitrogen  in  the  air  is  found  unchanged,  but  the 


CHEMISTRY,    ATOMS  205 

atoms  of  phosphorus  and  the   atoms   of  oxygen    have 
united  in  the  proportion  indicated. 

187.  Classification   of  Elements.— Elements   are   classi- 
fied as  metals  and  non-metals.     The  metals  possess  a  pe- 
culiar lustre  which  we  call  metallic.     They  are  good  con- 
ductors of  heat  and  electricity.   When  they  are  substituted 
for  the  hydrogen  in  acids,  they  form  salts.     When  chemi- 
cal action  takes  place  between  the  oxide  of  a  metal  and 
water,  a  new  compound  called  a  base  is  produced. 

The  non-metals  do  not  possess  a  metallic  lustre.  Most 
of  them  are  very  poor  conductors  of  heat  and  electricity. 
When  united  with  hydrogen,  they  generally  form  gaseous 
compounds.  When  chemical  action  takes  place  between 
the  oxide  of  anon-metal  and  water,  an  acid  is  produced. 

The  most  important  metals  and  non-metals  are  given 
in  the  table  in  §  182. 

The  meaning  of  some  of  the  terms  used  here  are  ex- 
plained in  the  next  section. 

188.  The    Classification    of   Compounds. — No   system   of 
classification  of  compounds  has  been  proposed,  up  to  the 
present  time,  which  can  in  any  sense  be  called  perfect. 
Nevertheless  the  classification  now  in  use  has  been  found 
to  be  very  convenient. 

The  four  most  important  classes  are  oxides,  acids,  bases, 
and  salts. 

An  oxide  is  a  combination  of  an  element  with  oxygen. 
H2O,  C02,  Mn02. 

An  acid  is  a  peculiar  combination  of  hydrogen  with  one 
or  more  other  elements.  The  combination  is  of  such  a 
character  that  the  hydrogen  may  be  more  or  less  readily 
displaced  by  a  metal,  so  that  the  hydrogen  compound  will 
be  changed  to  a  metal  compound. 

2HC1  +  Zn  =  ZnCl2  +  2H. 
H2SO4  +  Mg  =  MgSO4  +  2H. 


206  ELEMENTARY   PHYSICS 

Some  acids  are  formed  by  the  combination  of  hydrogen 
with  only  chlorine,  bromine,  iodine,  or  sulphur,  but  most 
acids  are  formed  by  the  union  of  hydrogen  with  two  or 
three  elements,  of  which  one  is  oxygen. 

A  base  is  formed  by  the  union  of  a  metal  with  one  or 
more  of  the  group  of  atoms  OH,  called  hydroxyl.  Those 
bases  which  contain  the  metals  sodium,  Na,  potassium, 
K,  or  the  group  of  atoms  NH4  are  also  called  alkalis, 
Na(OH),  K(OH).  NH4(OH). 

Acids  and  bases  have  several  exactly  opposite  proper- 
ties. Acids  have  an  acid  or  sour  taste ;  they  change  lit- 
mus-paper from  blue  to  red.  Bases  have  the  taste  of  lye, 
or  an  alkaline  taste,  and  they  change  litmus-paper  from 
red  to  blue.  Both  acids  and  bases  are  chemically  active 
substances,  but,  when  they  are  brought  in  contact,  they 
counteract  each  other,  and,  if  the  proportion  be  properly 
adjusted,  they  entirely  destroy  the  characteristic  prop- 
erties of  each  other.  They  neutralize  each  other.  In  the 
neutralized  solution  the  acid  is  no  longer  able  to  turn 
blue  litmus-paper  red,  and  the  base  cannot  turn  red  lit- 
mus-paper blue.  The  reason  for  the  inactivity  of  a  neu- 
tral solution  is  the  fact  that  in  a  neutral  solution  no  acid 
or  base  is  present.  As  soon  as  an  acid  and  a  base  come 
in  contact  with  each  other,  chemical  action  takes  place, 
the  metal  of  the  base  is  interchanged  with  the  hydrogen 
in  the  acid,  and  two  new  compounds  are  formed.  One  of 
these  compounds  is  water.  The  other  compound  is  a 
substance  called  a  salt. 

HC1  +  NaOH  =  NaCl  +  H2O. 

Salts  may  also  be  produced  by  chemical  reactions  in 
which  no  acids  are  present. 
A  salt  has  the  properties  of  neither  an  acid  nor  a  base. 


CHEMISTKY,   ATOMS  207 

It  is  a  neutral  substance.  It  has  exactly  the  same  com- 
position as  the  acid  from  which  it  was  derived,  except 
that  the  hydrogen  has  been  displaced  by  some  metal. 
The  name  salt  is  given  to  this  class  of  compounds,  be- 
cause most  of  these  compounds,  after  having  been  dis- 
solved in  water,  crystallize  when  the  water  is  evaporated 
and  the  crystals  of  the  great  majority  of  chemical  salts 
are  transparent  and  colorless  like  table  salt.  A  very  con- 
siderable number  of  chemical  salts,  however,  are  beauti- 
fully and  strongly  colored. 

189.  Poisons  and  Antidotes. — Both  acids  and  bases  are 
usually  poisonous,  at  least  when  taken  in  large  quanti- 
ties.    If  an  acid  has   been   swallowed,    some  base   like 
baking  soda  or  lime-water  is  given  to  neutralize  the  acid. 
If  the  substance  swallowed  be  a  base,   some  weak  acid, 
like  weak  vinegar  (acetic  acid),  will  serve  as  an  antidote. 
Different  poisons  require  different  antidotes,  but  the  prin- 
ciple involved  in    securing  an   antidote  to   a  chemical 
poison  is  usually  to  secure  a  chemical  which,  on  entering 
into  chemical  combination  with  the  poisonous  substance, 
will  form  a  harmless  compound.     Emetics  and  stomach- 
pumps  serve  to  remove  poisons  from  the  stomach,  and  if 
used  soon  enough  may  rid  the  stomach  of  poison  before 
much  has  been  taken  into  the  system. 

190.  Names   of  Compounds — When    oxygen,   chlorine, 
bromine,  iodine,  sulphur,  arsenic,  carbon,  all  of  which  are 
non-metallic  elements,  unite  with  only  one  other  element, 
and  that  element  is  a  metal,  the  compound  is  called  an 
oxide,  chloride,  bromide,  iodide,  sulphide,  arsenide,  or 
carbide  of  that  metal.     The  metal  is  usually  mentioned 
first.     For  instance,  PbI2  is  lead  iodide.     If  it  be  desira- 
ble to  mention  the  number  of  atoms  of  the  non-metallic 
element  present  in  the  compound,  the  following  prefixes 


208  ELEMENTARY   PHYSICS 

are  placed  before  the  name  of  the  non-metallic  element : 
mono  (one),  di  (two),  tri  (three),  tetra  (four),  penta  (five). 
For  example,  MnO2  is  manganese  dioxide  ;  PC13  is  phosr 
phorous  trichloride.  The  prefix  sesqui  means  one  and  a 
half.  It  is  used  when  the  ratio  of  the  metal  to  the  non- 
metal  is  2  to  3  (=1  to  1J).  For  instance,  Mn2O3  is  man- 
ganese sesquioxide. 

When  a  non-metallic  element  unites  with  hydrogen  to 
form  an  acid,  the  prefix  hydro  is  used  before  the  name  of 
this  non-metallic  element.  For  instance,  HC1  is  hydro- 
chloric acid. 

When  an  acid  consists  of  hydrogen,  oxygen,  and  some 
other  element,  this  other  element  gives  the  name  to  the 
acid  and  the  name  ends  in  ic.  For  instance,  HNO3  is  ni- 
tric acid. 

If  two  acids  are  formed  by  the  same  elements,  and  the 
number  of  atoms  of  oxygen  as  compared  with  the  number 
of  atoms  of  the  other  two  elements  is  not  the  same,  the 
name  of  the  acid  which  contains  the  smaller  quantity  of 
oxygen  ends  in  mis.  For  example,  H2SO4  is  sulphuric 
acid ;  H2SO3,  sulphurous  acid. 

When  three  elements  unite  to  form  three  acids,  each 
with  a  different  number  of  atoms  of  oxygen  in  the  mole- 
cule, the  one  containing  the  least  oxygen  has  the  prefix 
hypo  and  the  ending  ous  attached  to  the  name  of  the  acid. 
For  instance,  H3PO4  is  phosphoric  acid ;  H3PO3,  phos- 
phorous acid ;  and  H3PO2,  hypophosphorous  acid. 

Salts  produced  by  the  substitution  of  a  metal  for  the 
hydrogen  in  an  acid  are  named  by  using  both  the  name 
of  the  metal  and  the  name  of  the  acid.  But  if  the  name 
of  the  acid  ends  in  ic,  the  name  of  the  salt  ends  in  ate, 
and  if  the  name  of  the  acid  ends  in  ous,  the  name  of  the 
salt  ends  in  ite.  For  instance,  KNO3  is  potassium  ni- 


CHEMISTKY,    ATOMS  209 

trate ;  KNO2,  potassium  nitrite ;  BaSO4,  barium  sulphate  ; 
BaSO3,  barium  sulphite.  The  connection  between  the 
name  of  a  salt  and  that  of  the  acid  from  which  it  was  de- 
rived is  indicated  in  the  following  table: 


HN03      + 

Nitric  Acid 

KOH 

=      KN03      + 

Potassium  Nitrate 

H2O. 

HN02     + 

Nitrous  Acid 

KOH 

=      KNO2     + 

Potassium  Nitrite 

H20. 

H2SO4     + 

Sulphuric  Acid 

BaCl2 

=      BaSO4     + 

Barium  Sulphate 

2HC1. 

H2S03     + 

Sulphurous  Acid 

BaCl2 

=      BaS03     + 

Barium  Sulphite 

2HC1. 

The  endings  ic  and  ous  appended  to  the  names  of  the 
metals  in  the  names  of  compounds  call  attention  also  to 
the  quantity  of  oxygen,  chlorine,  or  non-metal  present. 
For  instance,  Fe2O3  is  ferric  oxide  ;  FeO,  ferrous  oxide ; 
FeCl3,  ferric  chloride  ;  FeCl2,  ferrous  chloride  ;  Fe2(SO4)3 
ferric  sulphate  ;  Fe2(SO4)2,  ferrous  sulphate. 

191.  Chemical  Action  Assisted  by  the  Presence  of  Water. 
— Chemical  action  does  not  take  place  between  lead 
nitrate  and  potassium  iodide  (§  171)  while  they  are  in 
the  solid  state.  Even  if  they  are  ground  together  and  in- 
timately mixed,  they  do  not  combine.  Each  particle  of 
solid  material  is  held  together  by  the  force  of  cohesion, 
and,  before  chemical  action  can  take  place,  this  cohesion 
must  be  overcome.  Dissolving  a  solid  in  water  loosens 
the  cohesion  between  the  molecules  ;  more  than  that,  it 
weakens  the  force  with  which  atoms  hold  together  in  the 
molecule.  The  result  is  that  chemical  action  will  often 
take  place  readily  between  solids  dissolved  in  water,  even 
in  cases  in  which  no  chemical  action  can  be  noticed  when 
these  solids  are  mingled  in  the  solid  state. 


210  ELEMENTARY   PHYSICS 

192.  Chemical  Action   Assisted  by   Heat. — Increase   of 
heat  causes  the  molecules  of  a  solid  to  move  farther  apart. 
Not  only  is  the  cohesion  between  the  molecules  lessened, 
but  also  the  force  with  which  the  atoms  in  each  molecule 
cling1  together  is  weakened.     In  this  condition  atoms  can 
enter  more  readily  into  combination  with  other  atoms. 
When  gunpowder  is  heated  beyond  a  certain  temperature, 
chemical  action  between  the  various   solids  used  takes 
place  with  such  violence  that  an  explosion  occurs. 

Chemical  action  between  solids  in  solution  also  is  very 
much  increased  by  heating  the  solution. 

193.  Heat  is   Consumed  during  the  Separation   of  Com- 
pounds into  Their  Elements  or  Components.— It  has  been 
shown  that  increase  of  heat  causes  molecules  to  move 
farther  apart.     This  is  true  not  only  in  cases  where  there 
is  no  change  of  state   (solids,  §  63 ;  liquids,  §  62 ;   arid 
gases,  §§  97,  115),  but  also  in  all  cases  of  change  of  state 
from  liquids  to  gases  (§§  96, 104),  and  in  most  of  the  cases 
of  change  from  solids  to  liquids  (§§  95,  104).     The  same 
fact  may  be  stated  in  another  form  by  saying  that  heat  is 
consumed  during  the  separation  of  molecules.    When  the 
molecules  of  solids  separate  on  dissolving  in  a  liquid, 
heat  is  consumed,  and,  since  the  heat  is  usually  taken  by 
the  solid  in  large  part  from  the  liquid  in  which  it  dis- 
solves, the  liquid  gets  colder  (§  114).     When  the  mole- 
cules of  liquids  separate  during  evaporation,  heat  is  con- 
sumed.    This  heat  is  secured  chiefly  from  the  air  into 
which  the  liquid  evaporates.     In  consequence,  the  air  be- 
comes colder  (§  113). 

According  to  the  same  principle  increase  of  heat  causes 
a  separation  of  atoms  (§§  134, 154)  already  in  combination 
as  molecules,  and  conversely  the  separation  of  atoms  from 
their  combinations  as  molecules  consumes  heat. 


CHEMISTKY,   ATOMS  211 

194.  When  Substances  Unite  to  Form  Compounds  Heat 
is  Produced. — It    has   been  shown  that,  when   gases  re- 
turn to  a  liquid  state,  or  when  liquids  return  to  a  solid 
state,  heat  is  given  out  although  there  is  no  fall  in  tem- 
perature (§§  111,  112).     It  is  also  a  matter  of  common  ex- 
perience that  hot  solids  and  liquids  give  out  heat,  when 
they  themselves  are  cooling  or  lowering  in  temperature. 
Experiments  have  been  devised  to  show  that,  while  sub- 
stances in  solution  are  crystallizing,  they  give  up  heat  to 
the  water  in  which  they  are  dissolved  or  that  the  tempera- 
ture of  the  water  rises. 

From  these  facts  the  general  rule  may  be  made,  that, 
while  the  molecules  of  substances  come  closer  together, 
they  give  up  heat  to  surrounding  bodies.  The  unusual 
behavior  of  water  has  so  far  not  been  explained  (§  117). 

The  coming  together  of  atoms  so  as  to  form  molecules 
also  produces  heat.  When  elements  unite  very  slowly 
or  in  small  quantities  at  any  one  time,  the  heat  produced 
may  pass  off  into  the  air  so  readily  that  it  may  not  even 
be  noticed.  However,  when  the  chemical  action  is  rapid, 
the  production  of  heat  can  be  detected  readily.  When 
water  is  poured  upon  a  piece  of  recently  burned  quick- 
lime, the  two  substances  unite  to  form  slaked  lime 
[CaO  -f  H2O  =  Ca  (OH)2]  and  the  heat  produced  is  con- 
siderable. This  action  may  be  seen  whenever  men  slake 
lime  to  make  mortar.  Fires  are  frequently  started  when 
water  gains  access  to  ships  or  warehouses  in  which  quick- 
lime is  stored. 

195.  Heat  may  be  Produced  with   Sufficient  Rapidity  to 
Cause  Light. — In  some  cases  the  chemical  combination  is 
sufficiently  rapid    to   cause    light  in   addition  to  heat. 
Straighten  a  piece  of  watch  spring.     Heat  the  tip  red  hot 
and  dip  it  into  sulphur  powder.     As  soon  as  the  sulphur 


212  ELEMENTARY   PHYSICS 

adhering  to  the  spring  has  ignited,  dip  the  spring  quickly 
into  a  jar  of  oxygen.  The  iron  will  unite  with  the  oxygen, 
and  sufficient  heat  will  be  produced  to  give  rise  to  a  brill- 
iant sputtering  light.  In  fact  the  light  caused  by  the 
ordinary  burning  of  objects  in  air  is  due  merely  to  the 
great  heat  produced  by  the  union  of  oxygen  with  the  ele- 
ments in  the  burning  material.  Ordinary  combustion  is, 
therefore,  due  to  rapid  union  of  the  elements  in  burning 
bodies  with  the  oxygen  in  the  air. 

Slow  combustion  is  vastly  more  common  than  ordinary 
combustion,  but  it  often  passes  unnoticed.  Most  of  the 
chemical  unions  that  take  place  in  nature  and  even  many 
of  those  which  take  place  in  the  laboratory,  take  place 
without  the  production  of  light.  In  many  of  these  cases 
oxygen  is  one  of  the  uniting  elements.  The  uniting  of 
oxygen  with  other  elements  is  called  oxidation. 

196,  The  Origin  of  Heat  in  the  Animal  Body One  of 

the  most  important  cases  of  slow  combustion  is  that  which 
is  the  cause  of  the  heat  in  the  animal  body.  The  heat- 
yielding  food-stuffs  that  enter  the  body  are  compounds 
which  always  contain  carbon,  hydrogen,  and  oxygen  (fat, 
sugar,  starch,  vegetables,  etc.)  and  often  also  nitrogen 
(meat).  The  materials  of  which  our  body  is  composed  con- 
sist of  the  same  elements.  When  the  compounds  in  our 
body  are  decomposed  they  unite  with  the  oxygen  brought 
in  through  our  lungs  and  circulating  through  our  blood. 
The  result  is  that  the  decomposition  of  the  compounds  in 
our  body  is  accompanied  by  a  slow  combustion  due  to 
the  union  of  oxygen  with  the  elements  which  formed 
part  of  these  compounds. 

Animal  heat  is  not  due  to  the  decomposition  of  the 
chemical  compounds  in  our  bodies,  since  the  separation 
of  atoms,  which  takes  place  during  decomposition,  is 


CHEMISTRY,    ATOMS  213 

accompanied  by  absorption,  not  by  emission  of  heat. 
But  the  heat  given  out  as  the  result  of  the  union  of  addi- 
tional oxygen  to  the  decomposition  products,  is  so  much 
greater  than  the  heat  absorbed  during-  the  separation  of 
the  atoms  in  decomposition,  that  on  the  whole  more  heat 
is  given  out  than  taken  up  and  the  heat  given  out  serves 
to  warm  the  animal  body. 


CHAPTEE  V 
SOUND 

197.  Sound  Produced  by  Vibrations. — Every  body  which 
produces  sound  is  in  motion.     When  a  tuning-fork,  violin, 
or  bell  produces  sound,  if  the  hand  touches  lightly  the 
prong-  of  the  tuning-fork,  the  string  of  the  violin,  or  the 
margin  of  the  bell,  the  motion  may  be  distinctly  felt.    If  a 
light  pith-ball  suspended  at  the  end  of  a  thread  is  brought 

into  contact  with  these  sounding 
bodies  (Fig.  87),  it  flies  off,  re- 
turns, and  flies  off  again,  as  if 
repeatedly  struck.  The  parts 
in  motion  move  short  distances 
only.  On  this  account  the  mo- 
tion is  often  not  visible  to  the  eye. 
The  parts  in  motion  move  to 
and  fro  like  the  pendulum  of  a 
clock,  but  with  greater  frequency. 
Even  the  least  frequent  of  these 
to  and  fro  motions  amount  to  at 

rIG.  »/. 

least  sixteen  and  a  half  per  sec- 
ond. This  frequency  makes  the  individual  motions  im- 
perceptible, even  if  the  distance  covered  is  great  enough 
to  be  visible  to  the  eye.  But  the  hazy  effect  produced  by 
these  rapid  motions  can  be  distinctly  noticed. 

198.  The  To  and  Fro  Motions  are   Called  Vibrations. — 
The  to  and  fro  motions  are  called  vibrations.     An  entire 
vibration  includes  both  the  to  and  the  fro  motion.   The  mo- 

214 


SOUND  215 

tion  in  one  direction  only,  from  the  farthest  point  reached 
in  the  right  to  the  farthest  point  in  the  left,  is  but  half  of 
a  vibration  (Fig.  88).  Half  of  the  motion  from  one  side  to 
the  other  is  called  the  amplitude  of  vibration. 
The  amount  of  time  taken  by  the  body  to  com- 
plete an  entire  vibration  is  called  the  period  of 
the  vibration.  The  number  of  vibrations  com- 
pleted by  the  body  during  one  second  is  called 
the  frequency  of  the  vibrations.  When  two  or 
three  bodies  complete  the  same  number  of  vi- 
brations per  second,  but  move  with  varying 
degrees  of  rapidity,  so  that  some  produce  vi- 
brations with  great  amplitudes,  and  other  vi- 
brations with  smaller  amplitudes,  the  vibra- 
tions are  said  to  vary  in  violence  or  intensity.  FIG  ^ 

199.  The  Frequency  of  the  Vibration  Deter- 
mines the  Pitch  of  the  Tone, — If  a  sounding  body  vibrates 
a  few  times  per  second,  it  will  give  a  low  tone.  If  it  vi- 
brates many  times  per  second,  it  will  give  a  high  tone. 
The  lowness  or  highness  of  a  tone  is  called  the  pitch  of 
the  tone.  The  frequency  of  the  vibrations  of  a  sounding 

body,  therefore,  de- 
termines the  pitch 
of  the  tone  which  it 
produces. 

This    may   be 
shown   easily  by  a 
FlG  89  simple  experiment. 

If  a  stiff  piece   of 

card-board  is  held  against  the  cogs  of  a  rapidly  turn- 
ing wheel  (Fig.  89)  taken  from  the  interior  of  some 
clock,  the  card-board  snaps  from  cog  to  cog,  being  first 
lifted  slightly  by  one.  cog,  and  then  allowed  to  slip  past 


216  ELEMENTARY  PHYSICS 

its  end  so  as  to  strike  the  next  cog.  This  causes  the 
end  of  the  card-board  to  move  up  and  down  or  to  vi- 
brate. As  long  as  the  wheel  moves  slowly  only  a  suc- 
cession of  snaps  is  heard,  but  as  soon  as  the  motion  is 
sufficiently  rapid  to  cause  the  card-board  to  snap  from 
cog  to  cog  at  the  rate  of  about  16  times  per  second,  a 
very  low  tone  is  heard  in  addition  to  the  snaps.  As  the 
motion  of  the  wheel  is  increased,  the  vibrations  produced 
by  the  card-board  are  more  frequent,  and  the  tone  heard 
in  addition  to  the  snaps  is  not  only  louder  but  also  higher 
in  pitch.  If  the  wheel  be  rotated  with  sufficient  frequency, 
the  pitch  may  rise  high  enough  to  be  disagreeable. 

200.  The  Frequency  of  Vibration  of  Various  Tones.— The 
lowest  C  of  the  pipe  organ  is  produced  by  16J  vibrations 
per  second,  the  lowest  A  of  the  grand  piano  by  27J  vi- 
brations per  second,  the  lowest  C  of  the  ordinary  piano 
by  33  vibrations,  and  the  lowest  E  of  the  double  bass 
violin  by  41J  vibrations.  The  highest  C  of  the  piano  is 
produced  by  4224  vibrations  per  second,  and  the  highest 
D  of  the  piccolo  flute  by  4752  vibrations  per  second. 
Tones  produced  by  vibrations  more  rapid  than  4752  per 
second  are  not  considered  useful  for  musical  purposes,  al- 
though vibrations  as  frequent  as  40,960  per  second  can  be 
heard. 

The  lowest  tones  which  are  audible  to  the  ear  are  used 
in  music  (pipe  organ),  although  it  is  questionable  whether 
they  have  any  distinct  musical  quality  when  the  vibra- 
tions number  less  than  32  per  second. 

The  frequency  of  vibration  of  a  mosquito's  wings  can 
be  determined  by  increasing  the  speed  of  the  wheel  of 
the  clock,  until  the  card-board  held  against  the  cogs 
gives  out  a  sound  of  equal  pitch.  From  the  number  of 
cogs  in  the  wheel  and  the  number  of  turns  per  second, 


SOUND  217 

the  number  of  vibrations  of  the  card  can  be  determined. 
Since  the  wings  of  the  mosquito  give  the  same  tone  as 
that  given  by  the  card,  the  number  of  vibrations  of  the 
wings  of  the  mosquito  must  be  the  same  as  the  number 
of  vibrations  of  the  card. 

In  the  same  manner,  the  number  of  vibrations  produced 
by  the  vocal  cords  in  the  throat  of  a  man  or  woman  sing- 
ing can  be  determined.  The  tone  of  G  written  on  the 
lowest  line  of  the  bass  staff  is  produced  by  96  vibrations 
per  second,  middle  C  by  256  vibrations,  and  F,  written  on 
the  highest  line  of  the  soprano  staff,  by  682.6  vibrations. 
It  is  possible  to  sing  both  lower  and  higher  than  these 
tones.  C  above  the  soprano  staff  is  produced  by  1024  vi- 
brations per  second. 

201.  A  Medium  is  Necessary  to  Transmit  the  Vibrations  of 
the  Sounding  Body  to  the  Ear. — Since  the  instrument  pro- 
ducing the  sound  and  the  ear  perceiving  the  sound  are 
always  at  some  distance  from  one  another,  something 
must  act  as  a  means  of  transmission  between  the  instru- 
ment and  the  ear.  This  medium  is  usually  the  air. 

An  alarm  clock  placed  under  a  bell- jar  on  the  plate  of 
an  air-pump  can  be  heard  easily,  but  if  the  air  is  pumped 
out  from  the  space  within  the  bell- jar,  the  sound  is  much 
less  distinct.  The  sound  could  not  be  heard  at  all,  if 
its  transmission  through  the  plate  of  the  air-pump  could 
in  some  way  be  prevented.  A  considerable  quantity  of 
cotton  batting  placed  under  the  clock  will  practically 
accomplish  this  (Fig.  90). 

"Water  also  may  serve  as  a  medium.  If  the  head  is 
held  under  water  while  some  one,  a  moderate  distance 
away,  pounds  together  two  stones,  also  held  beneath  the 
surface  of  the  water,  the  water  transmits  the  sound  so 
effectively  that  the  sensation  is  sometimes  even  painful. 


218 


ELEMENTARY   PHYSICS 


Even  solids  transmit  sound.  In  fact,  some  solids  trans- 
mit sound  better  than  air.  If  the  point  of  a  pin  is 
scratched  lightly  against  one  end 
of  a  long  wooden  pole,  the  sound 
can  be  heard  easily  by  a  person 
who  places  his  ear  against  the 
other  end.  However,  if  the  ear  is 
held  at  some  distance  away  from 
the  pole,  but  at  the  same  distance 
from  the  scratched  end  as  before, 
the  sound,  in  this  case  transmitted 
through  the  air,  may  not  be  audi- 
ble. The  cannonading  at  Antwerp 
in  1832  was  heard  in  the  mines  of 
Saxony  320  miles  away.  What  trans- 
mitted the  sound  ? 

202.  The  Sensation  of  Sound — 
The  original  cause  of  sound  and 
the  physiological  effect  of  sound 

are  two  widely  distinct  things.  The  original  cause  of 
sound  is  the  vibration  of  the  whole  or  of  a  part  of  some 
musical  instrument  or  other  body.  The  physiological 
effect  of  sound  is  the  sensation  produced  by  these  vibra- 
tions on  the  nerves  of  the  ear  (auditory  nerves).  The 
physicist  studies  the  original  cause  of  sound,  how  sound 
is  transmitted  from  the  original  sounding  body  to  the 
ear,  and  the  character  of  the  action  of  the  various  mech- 
anisms which  transmit  the  vibrations  from  the  air  to  the 
nerves  of  the  ear  (fibres  of  the  basilar  membrane,  §§  214- 
215).  The  physiologist  dissects  all  parts  of  the  organ  of 
hearing  and  studies  the  effect  produced  by  the  vibrations 
upon  the  auditory  nerves. 

If  there  is  110  air  present  to  transmit  these  vibrations, 


PIG.  90. 


SOUND  219 

if  there  is  no  ear  present  to  receive  them,  or  if  the  vibra- 
tions are  too  weak  to  affect  the  ear,  there  will  be  no 
sensation  of  sound.  Loudness  of  sound  is  the  intensity 
of  the  sensation  produced  in  the  nerves  of  the  ear.  Any 
cause  which  will  decrease  the  intensity  of  the  vibrations 
from  the  time  when  they  leave  the  sounding-  body  to  the 
time  when  they  reach  the  nerves  in  the  ear,  produces  a 
corresponding  decrease  in  the  loudness  or  strength  of 
the  sensation  in  the  auditory  nerves. 

203.  Sound  Transmitted  through  the  Air  by  Waves  of 
Condensation  and  Rarefaction. — When  the  prong  of  a  tun- 
ing-fork moves  forward,  it  condenses  the  air  immediately 
in  front  of  the  prong,  and,  as  the  molecules  strike  against 
molecules,  this  condition  of  condensation  travels  forward 
and  away  from  the  fork.  When  the  prong  swings  back 
during  the  second  half  of  its  vibration,  it  tends  to  leave  a 
vacuum  on  the  side  where  a  moment  before  it  had  pro- 
duced a  condensation.  As  the  nearest  air  particles  rush 
back  to  fill  up  this  space,  a  condition  of  rarefaction 
occurs  at  the  points  more  remote  from  the  fork,  from 
which  the  air  particles  have  returned.  This  condition  of 
rarefaction  is  taken  up  successively  at  points  more  and 
more  removed,  so  that,  while  the  molecules  of  the  air  are 
in  reality  moving  ~back  towards  the  fork,  the  condition  of 
rarefaction  may  be  said  to  be  spreading  outward  from  the 
fork.  Therefore,  both  the  condition  of  condensation  and 
that  of  rarefaction  travel  outward  from  the  fork. 

If  the  fork  continues  to  vibrate  for  some  time,  there  will 
be  a  regular  succession  of  these  condensations  and  rare- 
factions moving  away  from  the  fork.  Since  the  conden- 
sations are  produced  by  the  forward  motion  of  the  fork 
and  the  rarefactions  by  the  backward  motion,  the  rare- 
factions will  always  be  found  between  the  condensations. 


220 


ELEMENTARY   PHYSICS 


At  any  given  time  the  condensations  first  formed  will 
have  reached  points  in  the  air  more  distant  from  the  fork, 
while  the  condensations  or  rarefactions  later  formed  will 
not  have  travelled  so  great  a  distance  (Fig.  91). 

If  the  rate  of  vibrations  given  out  by  the  tuning-fork  be 
rapid,  the  distances  from  condensation  to  condensation 
will  be  short.  If  the  rate  of  vibrations  be  slow,  the  dis- 
tances between  successive  condensations  will  be  greater. 
As  long  as  the  rate  of  vibration  remains  the  same,  the 
distances  between  successive  condensations  will  be  equal. 

When  the  air  is  condensed  at  any  point,  it  spreads 
away  from  that  point  to  all  regions  where  the  pressure  is 


FIG.  91. 


less.  The  result  is  that  the  waves  of  condensation,  as 
they  leave  the  fork,  spread  in  all  directions  and  form  a 
succession  of  spherical  shells,  the  alternating  shells  being 
represented  by  alternating  conditions  of  condensation  and 
rarefaction  (Fig.  92). 

If  an  attempt  be  made  to  compare  the  series  of  conden- 
sations and  rarefactions  to  something  which  may  be  actu- 
ally seen  in  nature,  no  better  object  of  comparison  can  be 
found  than  the  succession  of  concentric  crests  and  hol- 
lows which  move  away  from  the  point  where  a  stone  is 
dropped  into  water.  For  this  reason,  the  successive  con- 
densations and  rarefactions  produced  in  the  air  by  sound- 
ing bodies  are  usually  referred  to  as  waves  of  sound. 


SOUND 


221 


204.  Although  the  Vibratory  Motion  is  Transmitted 
through  the  Air  for  Long  Distances,  the  Individual  Mole- 
cules of  Air  Move  Only  Short  Distances. — A  very  crude 
notion  of  the  action  of  the  molecules  during-  the  trans- 
mission of  sound  may  be  secured  from  the  following  ex- 
periments : 

If  a  dozen  boys  facing  in  the  same  direction  form  a  line, 


FIG.  92. 

each  boy  placing  his  hands  firmly  against  the  shoulders 
of  the  boy  in  front,  a  strong  push  against  the  boy  in  the 
rear  of  the  line  causes  the  boy  at  the  front  to  topple  over 
(Fig.  93).  The  motion  is  communicated  from  boy  to  boy. 
The  distance  along  which  the  motion  is  transferred  is 
much  greater  than  the  distance  passed  over  by  any  one 
boy. 


222 


ELEMENTARY   PHYSICS 


If  a  large  stone  be  dropped  into  a  pond,  a  series  of 
waves  will  travel  away  from  the  stone  to  the  margin  of  the 
pond.  Any  corks  floating'  upon  the  water,  however,  will 
merely  bob  up  and  down  as  the  waves  pass  beneath  them 
and  move  slightly  forward  and  backward  without  being 
•carried  permanently  nearer  the  shore.  It  is  evident  that 
waves  have  been  formed  in  the  water  without  any  con- 
siderable forward  motion  on  the  part  of  the  individual 
molecules  of  water  which  transmit  the  waves. 

In  the  same  manner  a  molecule  of  air  set  in  motion  by 
the  prong  of  a  tuning  fork  may  strike  against  the  mole- 


FlG.  93. 

cule  in  front,  this  molecule  may  strike  against  the  next 
molecule,  and  thus  the  motion  may  be  transferred  for 
great  distances  from  molecule  to  molecule  without  involv- 
ing any  great  amount  of  motion  on  the  part  of  any  one 
molecule. 

205.  The  Vibrations  of  the  Sounding  Body  Cause  the  In- 
dividual Molecules  of  Air  to  Vibrate. — A  more  exact  no- 
tion of  the  method  of  transmission  of  sound  through 
the  air  requires  a  study  of  the  character  of  the  motions 
of  the  individual  molecules.  When  the  prong  of  a  tuning- 
fork  moves  forward,  the  molecules  of  air  in  front  of  the 
prong-  also  move  forward,  and,  as  we  have  seen,  the  for- 


SOUND  223 

ward  motion  is  communicated  from  molecule  to  molecule, 
until  the  molecules  at  a  considerable  distance  from  the 
fork  also  move  forward.  When  the  prong  of  the  tuning- 
fork  returns  to  its  original  position,  it  tends  to  form  a 
vacuum  there  where  a  moment  before  it  produced  a  con- 
densation. The  pressure  of  the  air  in  contact  with  this 
side  of  the  prong  is  now  less  than  the  pressure  at  a 
greater  distance  from  the  fork.  The  molecules  of  air, 
which  a  moment  ago  were  moving  away  from  the  fork, 
may  now  return.  The  molecules  nearest  the  fork  return 
first.  This  diminishes  the  pressure  of  the  air  in  the  space 
from  which  they  return,  and  so  the  more  distant  mole- 
cules may  also  move  backward.  This  motion  is  taken  up 
by  molecule  after  molecule,  until  even  the  molecules  at  a 
considerable  distance  from  the  fork  have  taken  up  the 
returning  motion. 

As  the  prongs  of  the  tuning-fork  continue  to  move  for- 
ward and  backward,  a  corresponding  forward  and  back- 
ward motion  is  taken  up  by  all  the  molecules  of  the  air. 
The  molecules  nearest  the  fork  take  up  this  motion  first 
and  the  molecules  at  a  greater  distance  take  it  up  later. 
These  to  and  fro  motions  are  vibratory  motions,  and  it  is 
by  means  of  such  motions  that  sound  travels  through  the 
air. 

206.  Loudness,  or  Intensity  of  Sound. — The  intensity  of 
the  sensation  produced  in  the  ear  depends  not  directly 
upon  the  intensity  of  the  vibrations  of  the  molecules  in 
contact  with  the  sounding  body,  but  upon  the  intensity  of 
the  vibrations  of  those  molecules  which  come  in  contact 
with  the  ear.  If  the  molecules  which  come  in  contact 
with  the  ear  do  not  vibrate  strongly,  the  sound  will  be 
faint,  even  if  the  molecules  in  contact  with  the  vibrating 
body  are  in  violent  motion.  This,  however,  is  always  the 


224  ELEMENTARY  PHYSICS 

case  when  the  ear  is  at  a  considerable  distance  from  the 
sounding  body. 

When  the  air  is  condensed  at  any  point,  it  spreads 
away  from  that  point  to  all  regions  where  the  pressure  is 
less.  Therefore,  the  air  condensed  by  the  forward  motion 
of  the  tuning-fork  tends  to  spread  in  all  directions.  As 
the  condition  of  condensation  travels  outward,  it  spreads 
over  more  and  more  space,  and,  therefore,  the  degree  of 
condensation  becomes  less  as  the  distance  from  the  fork 
increases  (Fig.  92).  Since  these  condensations  are  pro- 
duced by  the  vibrations  of  the  molecules  of  the  air,  a 
lessening  of  the  amount  of  condensation  implies  a  lessen- 
ing of  the  amount  of  space  traversed  by  each  molecule 
during  its  vibrations.  Therefore,  the  intensity  of  the  vi- 
brations of  the  molecules  of  air  decreases  as  the  distance 
from  the  sounding  body  increases.  The  mathematical 
expression  for  this  statement  is  that  the  intensity  of  vibra- 
tion varies  inversely  as  the  square  of  the  distance. 

Anything  which  prevents  the  spreading  of  the  conden- 
sations after  they  leave  the  sounding  body  will  prevent, 
to  a  considerable  extent,  the  loss  in  the  intensity  of  vibra- 
tion as  the  vibratory  motion  is  transmitted  from  molecule 
to  molecule.  This  principle  is  made  use  of  in  speaking- 
tubes. 

In  an  indirect  manner,  of  course,  the  intensity  of  the 
sensation  of  sound  depends  not  only  upon  the  distance 
from  the  sounding  body  but  also  upon  the  intensity  of 
vibration  of  the  molecules  actually  in  contact  with  the 
sounding  body,  because,  if  these  molecules  do  not  vibrate 
strongly,  the  sound  will  be  weak  even  at  its  source,  and 
may  be  entirely  inaudible  at  a  short  distance  from  the 
sounding  body.  This  may  be  expressed  by  stating  that 
the  intensity  of  the  vibrations  of  the  molecules  in  contact  with 


•      SOUND  225 

the  ear  depends  upon  the  intensity  of  vibration  (amplitude  of 
vibration)  of  the  sounding  body. 

If  only  a  few  molecules  of  air  are  set  in  motion  by  the 
sounding1  body,  their  energy  is  soon  lost  in  causing"  the 
neighboring1  molecules  to  vibrate.  But  if  the  number  of 
molecules  originally  set  in  motion  is  very  great,  their  vi- 
brations will  be  transmitted  much  farther  before  they  be- 
come too  weak  to  affect  the  ear.  Therefore,  the  intensity 
of  vibration  of  the  molecules  in  contact  with  the  ear  de- 
pends not  only  upon  the  distance  of  the  ear  from  the 
sounding  body  and  the  intensity  of  vibration  of  the 
sounding  body  itself,  but  also  upon  the  number  of  mole- 
cules of  air  set  in  motion  by  direct  contact  with  the  sound- 
ing body.  If  a  tuning-fork  is  mounted  upon  a  box  not 
only  is  the  small  quantity  of  air  in  contact  with  the  fork 
set  in  motion  but  also  the  much  greater  quantity  in  con- 
tact with  the  box,  and  the  sound  will  be  audible  at  a  much 
greater  distance. 

These  facts  are  often  stated  in  the  form  of  the  following 
rules : 

Loudness  of  sound  varies  inversely  as  the  square  of  the 
distance. 

Loudness  varies  with  the  amplitude  of  vibration  of  the 
sounding  body. 

Loudness  varies  with  the  area  of  the  sounding  body. 

207.  The  Vibrating  Air  May  Communicate  Vibrations  to 
Other  Bodies, — Place  two  large  tuning-forks,  mounted  on 
boxes  and  producing  exactly  the  same  tone,  about  12  feet 
apart.  Direct  the  open  ends  of  the  boxes  toward  each 
other.  Strike  one  of  the  tuning-forks  once  or  twice  sharp- 
ly with  a  leather  mallet,  so  that  for  half  a  minute  it  gives 
out  a  loud  sound.  Then  grasp  the  prongs  of  this  fork  with 
the  hand  quickly,  so  that  the  fork  can  no  longer  vibrate. 


226  ELEMENTARY   PHYSICS 

If  the  ear  be  held  near  the  open  end  of  the  other  box 
(Fig.  94),  it  will  be  noticed  that  the  other  tuning-fork  has 
in  some  manner  been  set  in  vibration,  although  it  has  not 
been  struck  by  any  visible  material. 

The  first  tuning-fork  causes  the  air  in  contact  with  it 
to  vibrate.  This  vibratory  motion  is  transmitted  from 
molecule  to  molecule.  When  the  molecules  surrounding 
the  second  fork  move  forward,  they  push  the  prong  very 
slightly  in  the  same  direction,  and  when  a  moment  later 
the  molecules  move  in  the  opposite  direction,  this  prong 
of  the  tuning-fork  is  also  allowed  to  return. 

A  single  forward  and  backward  motion  of  the  air  sur- 


:EIG.  94. 

rounding  the  second  fork  does  not  make  this  fork  vibrate 
sufficiently  to  produce  an  audible  sound ;  but  the  accu- 
mulative effect  caused  by  a  regular  succession  of  a  large 
number  of  such  vibratory  motions  will  finally  cause  the 
second  fork  to  vibrate  sufficiently  to  produce  a  sound  loud 
enough  to  be  heard  if  the  ear  be  placed  near  the  fork. 

It  is  highly  essential  that  the  two  tuning-forks  should 
produce  the  same  tone ;  in  other  words,  that  they  should 
vibrate  at  exactly  the  same  rate.  Otherwise,  when  the 
vibrating  air  is  ready  to  return  at  the  end  of  its  first  half- 
vibration,  the  prong  of  the  tuning-fork  may  still  be  mov- 
ing forward,  and,  when  the  molecules  of  air  are  ready  to 


SOUND  227 

go  forward  the  second  time,  the  fork  may  not  have  com- 
pleted its  first  vibration.  In  this  case  there  is  not  suffi- 
cient agreement  between  the  vibrations  of  the  air  and  the 
vibrations  of  the  second  fork  to  enable  the  air  to  set  this 
fork  in  vibration  effectively. 

If  a  loud,  clear  note  be  sung  in  front  of  a  piano  with  the 
dampers  lifted  from  the  wire  (by  the  use  of  the  loud 
pedal),  the  wire  having  the  same  pitch  will  for  a  short 
time  give  out  the  same  tone. 

When  a  body  is  made  to  vibrate  by  vibrations  pro- 
duced in  the  air  by  some  other  body,  it  is  said  to  vibrate 
in  sympathy  with  the  other  body,  and  the  vibrations  are 
called  sympathetic. 

208.  Variations  in  the  Pitch  of  Strings. — Any  one  who  is 
familiar  with  the  appearance  of  the  interior  of  the  grand 
piano  knows  that  the  wires  of  the  piano  vary  in  length 
and  in  thickness.     Moreover,  while  some  strings  consist 
of  a  single  wire,  others  are  formed  by  wrapping  a  layer  of 
wire  around  a  thicker  central  wire  in  order  to  increase  the 
weight.     In  a  general  way  it  may  be  stated  that  the  short 
and  thin  wires  produce  the  higher  tones,  and  the  long, 
thick  or  heavy  wires  produce  the  low  tones.     Anything 
which  increases  the  weight  of  the  wire  lowers  the  tone. 
For  this  reason,  the  layers  of  wire  are  wrapped  around 
the  wires  intended  for  bass  tones.     The  more  tightly  a 
string  is  stretched,  the  higher  is  the  tone  which  it  pro- 
duces. 

209.  The  Number  of  Vibrations  Varies  Inversely  as  the 
Length  of  the  String.— The  number  of  vibrations  which  a 
string  will  produce  in  one  second  depends  upon  the  length 
of  the  string.     A  string  half  as  long  as  another  will  pro- 
duce twice  as  many  vibrations ;  one  a  third  as  long  will 
produce  three  times  as  many  vibrations  ;  one  a  fourth  as 


228 


ELEMENTARY  PHYSICS 


long  will  produce  four  times  as  many  ;  one  three-fourths 
as  long  will  produce  four-thirds  as  many  vibrations.  The 
mathematical  rule  is  that  the  number  of  vibrations  varies 
inversely  as  the  length  of  the  string,  provided  no  other 
change  be  made  in  the  string. 

210.  A  String  Vibrates  in  Parts  in  Addition  to  Vibrating 
as  a  Whole. — A  string  may  be  caused  to  vibrate  as  if  it 
were  a  series  of  separately  vibrating  strings  fastened  end 
to  end.  If  the  string  be  touched  lightly  at  the  middle, 
and  struck  at  the  same  time  at  a  point  J  of  the  length  of 


FIG.  95. 

the  string  frpm  the  end,  it  will  vibrate  in  two  sections  of 
equal  length.  If  the  string  be  touched  at  a  point  one- 
third  of  its  length  from  the  end,  and  struck  at  a  point  one- 
sixth  of  its  length  from  either  end,  it  will  vibrate  in  three 
sections  of  equal  length. 

Cause  it  to  vibrate  in  four  sections,  by  touching  it  at 
one-fourth  its  length  from  the  end.  Little  V-shaped 
pieces  of  paper  hung  across  the  wire  at  the  ends  of  the 
sections  will  not  be  thrown  off,  but  pieces  of  paper  placed 
at  the  middle  of  these  sections  will  fly  off  (Fig.  95).  This 
indicates  that  the  vibrations  are  strong  at  the  middle  of 


SOUND  229 

the  four  sections,  but  nearly  at  rest  at  the  ends  of  the  sec- 
tions. In  other  words,  the  string  is  vibrating  like  four 
sections  of  equal  length. 

A  string  vibrating  in  2,  3,  or  4  sections  vibrates  like 
several  strings,  each  J,  J,  or  J  as  long  as  the  entire  string. 
Each  section,  therefore,  now  produces  2,  3,  or  4  times  as 
many  vibrations  as  the  string  vibrating  as  a  whole  pro- 
duced. This  change  can  be  recognized  by  the  increase  in 
height  of  the  pitch,  as  the  string  is  made  to  vibrate  in 
more  numerous  sections.  A  string,  which,  when  vibrat- 
ing as  a  whole,  produces  C  on  the  bass  staff,  will  produce 
middle  C  on  the  piano,  when  it  vibrates  in  2  parts ;  G 
on  the  soprano  staff,  when  it  vibrates  in  3  parts ;  and  up- 
per C  on  the  soprano  staff,  when  it  vibrates  in  4  parts. 

A  string  can  be  made  to  vibrate  in  parts  at  the  same 
time  that  it  is  vibrating  as  a  whole.  Stretch  a  string  so 
that  it  produces  middle  C  when  vibrating  as  a  whole. 
Strike  it  strongly  at  a  point  one-sixth  of  its  length  from 
the  end.  You  will  hear  not  only  middle  C,  but  also  high 
G  above  the  soprano  staff,  showing  that  the  string  is  at 
the  same  time  vibrating  as  a  whole  and  also  in  three  sec- 
tions. 

211.  Instruments  Producing  the  Same  Fundamental  Tone 
are  Distinguished  by  Their  Overtones — All  musical  instru- 
ments vibrate  in  parts,  while  at  the  same  time  vibrating 
as  a  whole.  The  higher  tones  due  to  the  vibration  of 
the  instrument  in  parts  are  called  overtones.  The  tone 
produced  by  the  vibration  of  the  instrument  as  a  whole  is 
called  the  fundamental  tone.  Many  instruments  produce 
at  the  same  time  nine  or  more  overtones  of  different  pitch. 
Most  of  these  overtones  produce  pleasing  combinations 
with  the  fundamental  tone  and  increase  the  musical 
quality  of  the  sound  produced  by  the  instrument. 


230  ELEMENTARY  PHYSICS 

While  all  musical  instruments  produce  overtones  as  well 
as  fundamentals,  all  do  not  produce  the  same  overtones, 
or  at  least  do  not  produce  the  same  overtones  with  the 
same  degree  of  loudness.  Hence,  the  musical  effects  re- 
sulting from  the  combination  of  fundamentals  and  over- 
tones are  different  in  different  instruments.  This  makes 
it  possible  to  distinguish  different  kinds  of  instruments, 
even  when  they  are  producing  the  same  fundamental  tone. 

212.  Many  Vibrations  may  be  Transmitted  Through  the 
Air  at  the  Same  Time — One  might  suppose  that  it  would 
be    impossible    for    several    sets   of  vibrations   to   pass 
through  the  air  at  the  same  time  without  becoming  hope- 
lessly confused.     However,  that  this  is  not  true  is  shown 
by  every-day  experience.     The  vibrations  sent  out  by  the 
various  instruments  of  a  large  orchestra  all  travel  at  the 
same  time  through  the  air.     Yet  they  can  all  be  heard, 
and  a  skilful  conductor  can  at  once  pick  out  the  instru- 
ment which  makes  the  slightest  mistake.     In  the  same 
manner,  a  skilful  director  can  distinguish  the  individual 
voices  of  a  chorus. 

213.  The  Velocity  of  Sound. — The  velocity  of  sound  in 
air  at  the  ordinary  temperature  (72°  F.)  is  about  1130  feet 
per  second.     This  velocity  increases  and  decreases  at  the 
rate  of  1  foot  per  second  for  every  increase  or  decrease  of 
1  degree  in  the  temperature  of  the  air.     The  velocity  in 
water  is  4}  times  as  great ;  in  pine  wood,  10  times ;  and 
in  iron,  more  than  15  times  as  great  as  in  air.     Hence  the 
sound  of  a  blow  on  the  railroad  track  a  quarter  of  a  mile 
away  may  be  heard  coming  through  the  rail  more  than  a 
second  before  it  can  be  heard  through  the  air. 

214.  The  Structure  of  the  Ear. — For  the  purpose  of  de- 
scription, the  passage  between  the  opening  into  the  ear 
and  the  auditory  nerves  may  be  divided  into  three  parts. 


SOUND 


231 


— Ear  drum 


FIG. 


The  first  part  of  this  passage  is  a  tube  called  the  external 

auditory  canal.    It  is  about  one  and  a  quarter  inches  in 

length.    For  a  distance  of  about 

half  an  inch  from  the  opening 

into  the  ear,  the  canal  wall  is 

cartilaginous  and,  therefore, 

movable ;    for   the   rest   of  its 

extent,   about   three-fourths   of 

an    inch,    it    consists    of    bone 

covered  by  a  thin  skin. 

The  second  part  of  this  pas- 
sage is  called  the  tympanum  or 
middle  ear.  It  is  an  air-holding  cavity  of  irregular  shape 
hollowed  out  in  the  solid  bone  of  the  head.  It  is  about 
one-third  of  an  inch  long,  one-fourth  of  an  inch  high,  and 
one-sixth  of  an  inch  wide. 

Between  the  external  and  middle  ear  is  stretched  the 
membrane  known  as  the  ear  drum  (Fig.  96).     It  is  nearly 
circular  and  has  a  diameter  of  nearly  three-eighths  of  an 
inch.     Since  the  ear  drum  is  stretched  over  the  cavity  of 
the  middle  ear,  this  cavity  is  called 
the  tympanum  (Latin  for  drum). 

The  third  part  is  called  the  laby- 
rinth or  internal  ear.  It  is  a  liquid- 
holding  cavity  of  irregular  shape 
hollowed  out  of  the  solid  bone  (Fig. 
98).  The  lower  half,  occupying  the 
portion  nearer  the  inner  part  of  the 
head,  has  the  form  of  the  interior 

cavity  of  a  snail-shell,  and,  on  this  account,  is  called  the 
cochlea,  from  the  Latin  word  for  shell  (Figs.  98,  99) ;  it 
consists  of  two  and  one-half  spiral  turns.  The  upper  and 
more  exterior  half  consists  of  three  tubes,  called  semicir- 


Anvil 


Hammer 


%~  Stirrup 

Bones  of  the  Ear 
FIG.  97. 


232 


ELEMENTARY  PHYSICS 


Vestibule . 


Round 
Vfi 


Bony  labyrinth 
FIG.  98. 


cular  canals,  each  tube  bent  in  the  form  of  a  semicircle 
and  placed  at  right  angles  to  the  others.     The  open  ends 

of  the  semicircular  tubes  and 
of  the  snail-like  tube  all  open 
into  the  central  portion  of 
the  labyrinth. 

The  two  small  entrances 
from  the  middle  ear  to  the 
inner  ear  are  called  the  oval 
window  and  the  round  win- 
dow, and  are  also  each  cov- 
ered by  a  thin  sheet  of  mem- 
brane. Since  both  entrances 
to  the  internal  ear  pierce  the  walls  of  the  central  portion, 
this  part  is  called  the  vestibule  of  the  inner  ear.  The 
inner  ear  itself  is  called  the  labyrinth,  because  of  the 
bewildering  number  of  passages  it  contains. 

In  the  middle  ear,  between  the  ear  drum  and  the 
membrane  covering  the  oval  window,  is  a  series  of 
three  irregular  bones  (Figs.  96,  97).  To  the  first  bone 
attached  to  the  ear  drum  has  been  given  the  fanciful 
name  of  the  hammer,  and,  since  its  thickened  rounded 
head  rests  against  the 
second  bone,  this  bone  is 
called  the  anvil.  The  tip 
of  the  second  bone  is  at- 
tached to  the  end  of  a  third 
bone  which  looks  exactly 
like  a  minute  stirrup.  The 
flat  end  of  the  stirrup  is 
fastened  to  the  membrane 
stretched  over  the  oval  window  opening  into  the  ves- 
tibule of  the  inner  ear.  Any  motion  of  the  ear  drum 


Hound  window 
with  its  membrane 


Section  of  left  cochlea 
FIG.  99. 


SOUND 


233 


FIG.  100. 


is  at  once  transmitted  by  these  bones  to  the  oval  mem- 
brane. 

Within  the  snail-like  part  of  the  inner  ear,  the  cochlea, 
is  stretched  the  basilar  membrane  (Fig-.  100).  This  mem- 
brane consists  of 
16,000  to  24,000  mi- 
nute fibres,  which  are 
stretched  transverse- 
ly across  the  length 
of  the  passage  in  the 
cochlea.  These  fibres 
are  looked  upon  as  a 
sheet  of  parallel 
string's,  similar  to 
the  series  of  wires  in 
a  piano.  The  longer  fibres  are  found  nearer  the  smaller 
end  of  the  spiral  tube,  where,  at  first  thought,  they  would 
scarcely  be  expected.  These  fibres  are  weighted  with  the 
various  cells  of  Corti,  to  which  are  connected  the  auditory 
nerves. 

A.  large  part  of  the  structure  of  the  ear  has  purposely 
not  been  described.  A  fuller  knowledge  may  be  secured 
from  some  large  work  on  physiology  (American  Text 
Book  of  Physiology,  published  by  W.  B.  Sanders,  Phila- 
delphia.) 

215.  The  Communication  of  Sound  to  the  Nerves  of  the 
Ear. — When  the  molecules  of  air  set  in  motion  by  the 
vibration  of  some  other  body  strike  against  the  drum  of 
the  ear,  the  ear  drum  is  made  to  vibrate  at  the  same  rate 
as  the  molecules  of  air.  The  vibrations  of  the  ear  drum 
are  communicated  by  the  series  of  bones  in  the  middle 
ear  to  the  membrane  closing  the  oval  window  which  leads 
to  the  inner  ear.  The  membrane  transmits  the  vibra- 


234  ELEMENTARY  PHYSICS 

tions  to  the  liquid  filling  the  labyrinth.  Among  other 
places,  the  vibrations  reach  the  liquid  filling  the  snail- 
like  cavity,  or  cochlea.  Here  the  molecules  of  the  vibrat- 
ing liquid  are  in  contact  with  the  fibres  of  the  basilar 
membrane.  Those  fibres  of  the  membrane  which  vibrate 
naturally  at  the  same  rate  at  which  the  molecules  of  the 
liquid  in  the  cochlea  are  now  vibrating,  take  up  the 
vibrations  of  the  liquid.  These  vibrations  are  communi- 
cated then  to  the  cells  of  Corti,  which  weight  the  fibres, 
and  these  finally  produce  a  sensation  in  the  auditory 
nerves  connected  with  the  cells.  The  perception  of  this 
sensation  by  the  brain  is  called  hearing. 

Not  only  is  the  vibratory  motion  communicated  from 
body  to  body,  but  the  same  rate  of  vibration  is  taken  up 
in  succession  by  the  air,  ear  drum,  bones  in  the  middle 
ear,  membrane  covering  the  oval  window,  fluid  in  the 
cochlea,  and,  as  far  as  we  know,  also  by  one  or  several 
of  the  fibres  of  the  basilar  membrane. 

The  taking  up  of  the  vibrations  by  the  basilar  mem- 
brane is  another  case  of  sympathetic  vibration.  It  will 
be  remembered  that,  when  a  tone  is  sung  loudly  in  front 
of  the  piano,  that  wire  of  the  piano  which  possesses  the 
same  pitch  begins  to  vibrate  also. 


CHAPTER  YI 
BADTANT  ENERGY— HEAT  AND   LIGHT 

216.  Some  Vibrations  may  Produce  both  the  Sensation  of 
Heat  and  of  Light.— The  sensations  of  heat  and  light 
are  entirely  different  in  character  and  they  are  felt  by 
entirely  different  nerves.  The  nerves  of  temperature 
terminate  in  the  skin  and  extend  to  almost  every  part  of 
the  surface  of  the  body  (§  130).  The  nerves  of  sight,  how- 
ever, terminate  at  only  one  spot,  the  eye.  It  therefore 
might  reasonably  be  supposed  that  the  sensations  of  heat 
and  light  are  due  to  very  different  phenomena,  just  as 
the  sensations  of  sound  and  smell  are  unquestionably  due 
to  entirely  different  causes.  The  following  experiment, 
however,  suggests  that  the  sensations  of  heat  and  light 
may  be  due  to  exactly  the  same  phenomena. 

Place  a  piece  of  iron  in  a  very  hot  flame.  At  first  while 
it  rises  in  temperature,  it  gives  out  merely  more  and 
more  heat,  and  this  heat  may  be  felt  by  the  nerves  of 
temperature.  However,  after  a  certain  temperature  has 
been  reached,  the  iron  not  only  gives  out  heat,  but  also 
emits  light,  not  a  bright  light  but  at  least  a  light  that 
can  be  seen — a  dull  red.  This  suggests  that  if  the  in- 
crease in  temperature  of  the  iron  is  due  to  an  increase  in 
the  rate  and  intensity  of  vibration  of  the  molecules  of  the 
iron  (§  120),  that  light  may  be  due  to  more  rapid  vibrations 
and  that  slower  vibrations  are  not  able  to  produce  the 
sensation  of  light.  If  this  idea  is  correct,  the  general 

235 


236  ELEMENTAEY   PHYSICS 

statement  can  be  made,  that  a  great  variety  of  vibrations 
can  produce  a  sensation  in  the  nerves  of  temperature, 
but  only  the  more  rapid  vibrations  can  produce  any  sen- 
sation in  the  nerves  of  sight.  If  red  is  the  first  color  seen 
when  a  body  is  raised  in  temperature,  then  all  the  other 
colors  should  also  be  produced  by  the  more  rapid  vibra- 
tions. This  has  been  shown  to  be  true,  but  not  by 
means  of  the  simple  experiment  here  described. 

If  the  temperature  of  the  iron  be  increased  above  that 
necessary  to  produce  the  dull  red  color,  the  iron  will  give 
out  a  brighter  red,  then  a  reddish-orange,  later  a  yellow, 
and  finally  a  white  color.  In  following  paragraphs  it  will 
be  shown  that  many  other  colors  are  given  out,  but  that 
these  are  so  mingled  together  that  until  we  learn  how  to 
separate  them,  we  are  not  able  to  recognize  their  presence 
(§.  231).  For  the  present  it  will  be  sufficient  to  recognize 
the  fact  that  vibrations  of  many  degrees  of  rapidity  may 
produce  the  sensation  of  heat,  but  that  only  the  more 
rapid  vibrations  can  produce  the  sensation  of  light. 

217.  The  Transmission  of  Heat  by  Contact  of  Molecule 
with  Molecule. — Heat  may  pass  from  one  part  of  a  body 
to  another,  by  the  communication  of  heat  from  molecule 
to  molecule.  When  the  molecules  do  not  change  their 
relative  positions,  as  in  the  case  of  solids,  this  method  of 
transference  of  heat  is  called  conduction  (§  121).  When 
the  molecules  change  their  relative  positions,  as  in  the 
case  of  liquids  and  gases,  it  is  called  convection  (§§  125, 
127).  In  both  cases  heated  molecules  come  in  contact 
with  others  not  yet  heated,  and  communicate  to  them  a 
part  of  their  own  heat. 

When  water  is  boiled  in  a  kettle,  the  hot  molecules  in  the 
flame  communicate  a  part  of  their  heat  to  the  molecules 
of  iron  forming  the  exterior  of  the  kettle.  This  heat  is 


RADIANT   ENERGY— HEAT   AND   LIGHT      237 

communicated  from  molecule  to  molecule  until  it  reaches 
the  interior  of  the  kettle.  Here  it  is  transmitted  to  the 
molecules  of  water  in  contact  with  the  kettle,  and,  as  these 
move  away,  other  molecules  of  water  may  take  their 
places  and  also  become  heated. 

218.  The  Air  does  not  Transmit  the  Heat  or  Light  from 
the  Sun  to  the  Earth. — Heat  may  also  be  transmitted  from 
molecule  to  molecule  without  direct  contact,  and  without 
any  transmission  by  means  of  an  intermediate  set  of  mole- 
cules.    During  a  hot  cloudless  summer  day  the  tempera- 
ture at  the  surface  of  the  earth  rises  frequently  to  100°  F. 
However,  as   one  ascends  to  regions  above  the  earth,  a 
steady  lowering  of  the  temperature  is  observable.     Near 
the  surface  of  the  earth  the  temperature  decreases  at  the 
average  rate  of  1°  F.  for  every  increase  in  elevation  of 
about  300  feet.   At  an  elevation  of  several  miles  above  the 
earth  the  rate  of  decrease  of  temperature  is  considerably 
less.     This  is  shown  by  the  fact  that  Glaisher  found  a 
total  decrease  in  temperature  of  only  61°  F.  when  he  as- 
cended in  a  balloon  to  a  height  of  5  miles.     Nevertheless 
we  have  every  reason  to  believe  that  at  a  distance  of  100 
miles  above  the  surface  of  the  earth,  the  temperature 
must  be  nearly  460°  below  zero,  Fahrenheit.     Since  the 
temperature  of  the  air  at  the  surface  of  the  earth  is  higher 
than  the  temperature  of  the  air  nearer  the  sun,  it  is  evi- 
dent that  the  heat  and  light  of  the  sun  reach  the  earth 
without  being  conducted  to  the  earth  by  the  air. 

219.  No  Air  is  Present  in  the  Greater  Part  of  the  Space 
Between  the  Sun  and  the  Earth. — The  density  of  the  air 
diminishes  as  the  elevation  above  the  surface  of  the  earth 
increases.     At  an  elevation  of  If  miles  the  air  is  not  f  as 
dense  as  at  sea.  level,  at  3|  miles  it  is  less  than  half  as 
dense,  at  6  miles  it  is  less  than  J  as  dense  (§  17).     It  is 


238  ELEMENTARY   PHYSICS 

evident  that  100  miles  above  the  surface  of  the  earth  the 
air  must  be  exceedingly  rare. 

How  do  we  know  that  air  of  any  degree  of  density  is 
present  100  miles  above  the  earth  ?  Tiny  particles  of  rock 
flying  through  space  frequently  come  in  contact  with  the 
atmosphere  of  the  earth.  Since  the  speed  with  which 
the  earth  moves  is  about  18.5  miles  per  second,  and  since 
the  particles  of  rock  are  also  moving  rapidly,  the  total 
speed  with  which  the  particles  of  rock  and  the  earth  ap- 
proach each  other  may  amount  to  as  much  as  45  miles  per 
second.  When  the  particles  of  rock  strike  the  molecules 
of  the  air,  the  heat  produced,  as  a  result  of  the  concus- 
sions, is  so  great  that  both  air  and  rock  become  white  hot 
and  are  then  visible  in  the  form  of  shooting  stars.  When 
larger  masses  of  rock  strike  the  air,  they  not  only  give  out 
a  much  more  brilliant  light,  but  occasionally  portions  of 
the  rock  reach  the  surface  of  the  earth,  and  are  then 
called  meteorites.  When  two  observers  in  neighboring 
towns  notice  the  same  meteor  and  record  the  angle  at 
which  it  appears  above  the  horizon,  it  is  possible  to  deter- 
mine the  elevation  of  the  meteor,  as  soon  as  the  distance 
between  the  two  towns  has  been  measured.  Observations 
of  a  similar  kind  indicate  that  the  tiny  particles  of  rock 
which  become  shooting  stars  occasionally  become  visible 
at  elevations  between  70  and  100  miles  above  the  surface 
of  the  earth.  Therefore  at  100  miles  above  the  surface 
of  the  earth  some  air  is  still  present. 

Since  no  shooting  stars  ever  appear  at  a  greater  eleva- 
tion, it  is  evident  that  at  still  higher  elevations  the  quan- 
tity of  air  continues  to  decrease  until  finally  air  is  prac- 
tically absent.  By  far  the  greater  part  of  the  space  be- 
tween the  earth  and  the  sun  cannot  contain  air.  But 
if  air  is  present  for  only  a  short  distance  between  the 


RADIANT   ENERGY— HEAT   AND   LIGHT      239 

sun  and  the  earth,  then,  along  most,  if  not  all,  of  the  dis- 
tance between  the  sun  and  the  earth,  the  heat  and  light 
of  the  sun  must  be  transmitted  by  means  of  some  other 
medium  than  air. 

220.  Heat  and  Light  may  be  Transmitted  Through  a 
Vacuum. — The  possibility  of  the  transmission  of  heat  and 
light  through  a  vacuum  may  be  demonstrated  experimen- 
tally. 

Enclose  the  blackened  bulb  of  a  thermometer  within  a 
glass  globe.  By  means  of  a  mercury-pump  withdraw  the 
air  which  is  in  the  globe,  and  melt  shut  the  opening 
through  which  the  air  was  pumped  out  (Fig.  101).  By 


FIG.  101. 

means  of  this  type  of  pump  it  is  possible  to  get  an  almost 
perfect  vacuum.  Less  than  a  millionth  of  the  air  is  left 
behind. 

If  a  red-hot  poker  be  held  near  the  globe,  the  mercury 
in  the  thermometer  instantly  rises.  Since  the  interior  of 
the  globe  is  practically  a  vacuum,  air  cannot  have  served 
as  a  medium  for  the  transmission  of  heat,  at  least  in  the 
space  between  the  walls  of  the  globe  and  the  bulb  of  the 
thermometer.  The  part  of  the  thermometer  enclosed  in 
the  globe  can  be  seen  as  readily  through  the  vacuum  as 
before  the  removal  of  the  air.  Therefore,  light  can  also 
pass  through  a  vacuum. 


240  ELEMENTARY   PHYSICS 

221.  In  Order  to  Explain  the  Transmission  of  Heat  and 
Light  Through  Space  in  which  no  Material  Substance  Is 
Known  to  Be  Present,  the  Existence  of  Ether  Is  Assumed, 
— It  is  impossible  to  conceive  of  the  transmission  of  heat 
from  the  sun  to  the  earth  without  the  existence  of  some 
intervening  medium.  For  this  reason  scientists  have 
proposed  the  theory  that  all  space  which  is  apparently 
empty  is  in  reality  filled  with  something  which  they  call 
the  ether,  and  they  have  suggested  that  in  all  cases  in 
which  heat  is  not  transferred  by  some  known  medium,  in 
other  words,  by  contact  of  molecule  with  molecule,  the 
heat  is  transferred  in  some  manner  by  the  ether.  Know- 
ing what  phenomena  of  heat  and  light  it  was  necessary  to 
explain  by  means  of  the  ether,  scientists  then  proceeded 
to  determine  what  must  be  the  properties  of  the  ether,  in 
order  that  it  might  show  these  phenomena. 

They  soon  came  to  the  conclusion  that  the  ether  must 
be  colorless,  inconceivably  light  in  weight,  more  elastic 
than  any  substance  actually  known  to  exist,  and  so  all- 
pervading  that  it  not  only  occupies  all  space  ordinarily 
considered  as  a  vacuum,  but  even  fills  the  space  between 
the  atoms  of  ordinary  matter.  It  is  therefore  believed  to 
be  vastly  more  continuous  than  anything  actually  seen  on 
earth ;  for  the  molecules  of  even  the  densest  substances 
are  known  to  be  not  in  actual  contact  with  each  other, 
since  every  decrease  in  temperature  at  once  brings  them 
closer  together.  This  ether  can  be  set  in  vibration  by  the 
vibrations  of  the  atoms  imbedded  within  the  ether.  It 
can  transmit  these  vibrations  to  more  distant  parts  of  the 
ether,  and  it  can  communicate  this  vibratory  motion  to 
other  atoms  at  a  distance  from  those  atoms  which  origi- 
nally set  the  ether  in  motion. 

This  theory  at  first  rested  chiefly  upon   imagination, 


RADIANT   ENERGY,   HEAT   AND   LIGHT     241 

and  was  merely  an  attempt  to  explain  many  phenomena 
otherwise  inexplainable.  However,  within  recent  years 
this  theory  has  been  found  capable  of  explaining  so  many 
phenomena  in  heat,  light,  and  electricity,  that  the  exist- 
ence of  ether  may  be  considered  as  proved.  Some  scien- 
tists have  even  attempted  to  determine  approximately  its 
density  and  elasticity  (Principles  of  Physics,  Daniell. 
Macmillan  &  Co.,  New  York). 

A  liquid  known  as  ether  is  sold  in  drug  stores.  The 
ether  which  transmits  light  has  no  connection  with  this 
liquid.  The  ether  known  in  theory  is  entirely  invisible, 
and,  if  it  were  not  for  the  phenomena  of  heat,  light,  and 
electricity,  its  existence  would  never  have  been  suspected. 

222.  Radiation,— When  a  pebble  falls  into  a  pool  of 
water,  it  starts  a  series  of  waves.  The  waves  spread  out 
in  every  direction  and  set  in  motion  every  light  object 
floating  upon  the  water. 

According  to  the  ether  theory  vibrating  atoms  start 
waves  in  the  ether  surrounding  each  atom.  These  waves 
are  transmitted  by  the  ether  in  every  direction  and  strike 
other  atoms  imbedded  in  other  parts  of  the  ether.  In  con- 
sequence these  more  distant  atoms  also  begin  to  vibrate. 

The  ether  theory  may  be  used  to  explain  the  transmis- 
sion of  heat  and  light  from  the  sun  to  the  earth  without 
the  use  of  air.  The  atoms  forming  the  sun  vibrate  at  an 
exceedingly  rapid  rate.  This  produces  a  vibratory  motion 
in  that  part  of  the  ether  surrounding  each  atom  and  gives 
rise  to  waves  traversing  the  ether.  These  waves  pass 
through  the  enormous  spaces  devoid  of  air  but  filled  with 
ether.  On  reaching  the  atmosphere  surrounding  the 
earth,  the  wave  motion  of  the  ether  gives  rise  to  vibratory 
motion  in  the  atoms  forming  the  air.  At  the  surface  of 
tbe  earth  they  cause  the  atoms  present  in  the  rock-forming 


242  ELEMENTARY   PHYSICS 

compounds  to  vibrate,  since  these  also  are  imbedded  in 
the  ether.  In  fact,  the  atoms  of  all  substances  at  the  sur- 
face of  the  earth  are  more  or  less  affected,  since  ether 
pervades  all  space,  even  the  space  between  the  atoms  of 
the  densest  substances. 

The  vibratory  motion  of  the  atoms  gives  rise  to  the 
phenomena  known  as  heat.  Some  atoms  take  up  the 
vibratory  motion  more  readily  than  others,  and  therefore 
become  hotter.  On  this  account  rocks  become  hotter 
than  air.  When  the  rate  and  intensity  of  the  vibrations 
communicated  to  the  atoms  are  sufficient,  the  vibratory 
motion  of  the  atoms  gives  rise  to  both  heat  and  light.  In 
proportion  as  the  ether  sets  the  atoms  imbedded  within  it 
into  more  violent  vibration,  the  intensity  of  its  own  mo- 
tion decreases.  Hence  the  heating  effects  of  the  sun  do 
not  penetrate  very  far  beyond  the  surface  of  objects  struck 
by  its  rays. 

The  transmission  of  vibrations  capable  of  producing 
the  sensation  of  heat,  or  of  both  heat  and  light,  is  called 
radiation. 

223.  Ether  may  Serve  to  Transmit  Heat  and  Light  even 
when  Air  is  Present. — If,  on  a  cold  winter  day  before 
the  room  has  become  warm,  we  stand  before  a  large  fire 
in  an  open  fireplace,  the  heat  striking  the  body  may  be 
so  great  that  it  even  scorches  us  ;  but  if  a  large  screen 
be  placed  between  us  and  the  fire,  we  will  at  once  feel 
cold.  Yet  we  know  by  experience  that  air  does  not  be- 
come cold  instantly  when  the  source  of  heat  is  removed. 
Is  the  air  between  us  and  the  fire  actually  warm  ? 

If  we  place  a  delicate  thermometer  behind  a  piece  of 
wood  which  will  serve  as  a  screen  for  the  thermometer, 
and  then  hold  it  thus  protected  between  us  and  the  fire, 
we  will  learn  that  the  temperature  of  the  air  is  still  low. 


RADIANT  ENERGY— KEAT   AND   LIGHT     243 

However,  the  instant  the  thermometer  is  removed  from 
behind  the  piece  of  wood  and  allowed  to  face  toward  the 
fire,  the  temperature  rises  considerably.  Those  parts  of 
our  bodies  which  face  the  fire  also  feel  much  more  heat 
than  can  be  accounted  for  by  the  actual  temperature  of  the 
intervening  air,  as  indicated  by  the  thermometer.  The 
vibrations  of  the  heated  atoms  in  the  fire  are  transmitted 
by  means  of  the  ether  which  is  between  the  atoms  form- 
ing the  air.  As  soon  as  the  waves  set  up  in  the  ether  in- 
crease sufficiently  the  rate  of  vibration  of  the  atoms  of 
mercur\r,  the  thermometer  indicates  a  rise  in  temperature. 
Waves  of  a  still  higher  rate  of  vibration  may  produce  both 
the  sensation  of  heat  and  of  light. 

224.  Refraction  of  Light  Passing  Obliquely  from  Water 
to  Air, — The  waves  set  up  in  the  ether  spread  in  every 
direction  from  the  centre  of  disturbance.  Each  wave 
may  be  looked  upon  as  a  spherical  shell  enlarging  with 
enormous  rapidity.  When  waves  strike  obstructions 
only  the  unobstructed  parts  of  waves  may  continue  their 
progress.  Hence  light  passes  through  rifts  in  clouds  in 
long  sheets  or  beams.  Through  smaller  apertures  the  wave 
continues  its  course  in  the  form  of  rays.  Since  every 
part  of  a  wave  travels  in  a  radiate  manner  directly  away 
from  the  centre  of  disturbance,  that  part  of  the  wave  which 
continues  its  path  through  a  small  aperture  is  straight, 
at  least  as  long  as  the  substance  through  which  it  passes 
has  the  same  density.  Hence  rays  of  light  are  straight. 

However,  when  a  ray  travels  through  substances  hav- 
ing different  degrees  of  density,  the  path  followed  by  the 
ray  changes  at  each  point  where  a  new  substance  is  en- 
tered. The  bending  of  a  ray  of  light  on  passing  from  one 
substance  into  another  can  be  shown  easily  by  the  fol- 
lowing experiment. 


244 


ELEMENTAKY  PHYSICS 


Place  a  coin  in  the  centre  of  a  pan  set  on  a  low  table. 
The  sides  of  the  pan  should  be  nearly  perpendicular,  and 
almost  4  inches  high.  Take  a  position  at  such  a  distance 
that,  when  you  glance  across  the  edge  of  the  pan,  only 
the  farther  margin  of  the  coin  is  visible  (Fig.  102).  The 
light  from  the  coin  now  passes  in  a  straight  line  through 
the  air  past  the  edge  of  the  pan  to  the  eye.  Now  let 


FIG.  102. 

some  one  fill  the  pan  with  water  up  to  the  brim,  and  then 
lower  the  head  until  the  light  from  the  farther  margin 
of  the  coin  is  again  seen  just  above  the  edge  of  the  pan 
(Fig.  103).  The  path  of  the  light  is  now  partly  in  water 
and  partly  in  air.  That  part  of  the  path  which  extends 
from  the  farther  margin  of  the  coin  to  the  edge  of  the 
pan  passes  in  a  straight  line  through  the  water.  The  re- 


RADIANT   ENERGY— HEAT   AND   LIGHT     245 

maiuder  of  the  path,  from  the  edge  of  the  pan  to  the  eye, 
passes  in  a  straight  direction  through  the  air.  But  the 
eye  is  no  longer  in  its  former  position.  The  path  of 
light  has  been  evidently  bent  downward  at  the  surface 
of  the  water  near  the  margin  of  the  pan.  This  bending 
of  the  light  while  entering  another  substance  at  an  ob- 
lique angle  is  called  refraction. 

If  the  coin  be  watched  while  the  basin  is  filling  with 
water,  it  will  be  noticed  that  the  coin  comes  into  view 
and  seems  to  move  upward  and  toward  the  centre  of  the 


FIG.  103. 


pan  as  the  water  increases  in  depth.  This  effect  is  pro- 
duced by  the  bending  of  the  rays  of  light  as  they  pass 
from  the  surface  of  the  water  out  into  the  air.  By  further 
study  it  may  be  determined  which  rays  are  bent,  what 
was  their  former  course,  and  why  the  coin  seems  to  move. 
The  only  purpose  of  the  preceding  experiment,  however, 
is  to  bring  out  the  fact  that  light  is  bent  downward  on 
passing  obliquely  from  water  into  air. 

225.  Refraction  of  Light  which  Passes  Obliquely  from  Air 
into  Glass,  and  from  Glass  into  Air. — Secure  from  a  dealer 
in  physical  apparatus  a  rectangular  piece  of  plate-glass, 


246 


ELEMENTARY   PHYSICS 


5  inches  long  and  3  inches  wide,  whose  long-,  narrow 
sides  have  been  ground  so  as  to  be  flat  and  parallel, 
and  then  polished  so  as  to  be  almost  perfectly  trans- 
parent.  Lay  the  glass  on  the  table,  several  inches  from 
the  margin.  Stick  a  pin  into  the  table,  several  inches 
behind  the  plate  of  glass,  and  then  lowering  the  head  to 
the  level  of  the  table,  look  at  the  pin  through  the  entire 
width  of  the  glass.  Move  the  head  toward  one  side 
until  the  lower  part  of  the  pin  as  seen  through  the  glass 


FIG.  104. 

is  very  far  out  of  line  with  the  upper  part  of  the  pin  as 
seen  through  the  air,  above  the  glass.  Then,  without 
moving  either  the  plate  or  the  eye,  stick  a  second  pin 
into  the  table,  in  contact  with  the  more  distant  of  the 
polished  faces  of  the  glass,  so  as  partly  to  hide  the  lower 
part  of  the  first  pin  looked  at  through  the  glass  (Fig.  104). 
This  will  mark  the  point  where  the  light  from  the  first 
pin  enters  the  glass.  Stick  a  third  pin  into  the  table,  in 
contact  with  the  nearer  polished  face,  so  as  to  hide  the 


RADIANT   ENERGY— HEAT   AND   LIGHT     247 

lower  part  of  the  second  pin.  This  will  indicate  where 
the  light  from  the  first  pin  would  leave  the  glass  and 
re-enter  the  air  if  not  obstructed  by  the  second  pin. 
Finally,  stick  a  fourth  pin  in  the  table,  several  inches  in 
front  of  the  plate,  so  as  to  hide  the  third  pin,  which  is  of 
course  in  plain  p-iew.  This  will  indicate  in  what  direc- 
tion the  light  from  the  first  pin  would  pass  on  leaving 
the  glass  if  not  obstructed  by  the  other  pins. 

If  the  position  of  the  head  has  not  been  changed,  all 
four  pins  appear  to  be  in  a  straight  line  while  the  ob- 
server is  looking  through  the  entire  width  of  the  glass. 
However,  if  the  head  is  lifted  and  the  pins  are  connected 


in  their  order  by  straight  lines,  it  will  be  found  that  the 
light  from  the  first  pin  bends  both  on  entering  the  glass 
and  on  re-entering  the  air,  but  the  bending  in  the  two 
cases  is  in  opposite  directions. 

226,  A  Small  Portion  of  a  Spherical  Wave  is  Practically 
Flat. — The  cause  of  the  bending  of  the  rays  of  light  in 
passing  from  one  substance  into  another  may  be  explained 
in  the  following  manner  : 

The  vibrations  of  atoms  at  the  source  of  heat  and  light 
produce  spherical  waves  which  spread  through  the  ether 
in  all  directions  from  the  centre  of  disturbance  outward. 
In  their  spherical  form  they  resemble  the  waves  of  sound 


248  ELEMENTARY  PHYSICS 

(§  203).  At  a  considerable  distance  from  the  original 
cause  of  disturbance  the  curvature  of  any  small  portion 
of  the  front  of  a  wave  is  very  small  (Fig-.  105).  There- 
fore, any  small  part  of  the  wave  front  may  be  treated,  in 
a  theoretical  discussion,  as  if  it  were  flat. 

It  may  be  interesting1  in  this  connection  to  notice  that 
many  people  still  believe  that  the  surface  of  the  earth  is 
flat,  because  the  small  portions  of  its  surface  which  they 
can  see  at  any  one  time  often  appear  flat. 

227.  Refraction  of  Waves  Due  to  Variation  in  Amount  of 
Speed  of  Waves  while  they  are  Passing  through  Different 
Substances. — If  a  thick  flat  piece  of  glass  is  held  in  front 
of  a  wave  advancing  through  the  ether  in  the  air,  a  por- 
tion of  the  wave  will  enter  the  glass.  If  the  glass  be  held 
obliquely,  that  end  of  the  glass  which  is  nearer  to  the 
wave  front  will  be  entered  first. 

It  has  been  found  that  light  does  not  travel  as  fast 
through  glass  as  it  does  through  air.  Consequently  the 

velocity  of  that  part  of 
the  wave  which  enters 


the  glass  first  will  be  re- 
tarded (Fig.  106),  while 
the  remainder  of  the 
wave  continues  to  travel 
with  its  original  veloc- 
ity, until  it  also  enters 

Air  the  glass.  This  permits 

the  lagging  end  of  the 
FJG  IQ6  wave  to  catch  up  partly 

with  that  end  which  en- 
ters the  glass  first.  In  consequence  the  wave  front  swings 
around  so  that  it  is  more  nearly  parallel  to  the  surface  of 
the  glass,  after  it  has  entered  the  glass,  than  it  was  before. 


RADIANT   ENERGY— HEAT   AND   LIGHT     249 

As  soon  as  the  entire  wave  front  lias  entered  the  glass 
there  is  no  further  change  in  direction,  since  all  parts  of 
the  wave,  while  in  the  glass,  travel  with  the  same  veloc- 
ity. Since  each  part  of  a  wave  always  moves  in  the  di- 
rection in  which  the  wave  front  faces,  it  is  evident  that 
the  wave,  on  entering  the  glass,  will  take  up  a  path 
which  is  different  in  direction  from  its  path  in  air. 

The  mathematical  statement  of  this  change  of  direction 
is  that  light  bends  toward  the  perpendicular  in  passing 
from  air  into  glass.  The  perpendicular  is  supposed  to  be 
erected  at  the  point  where  the  light  enters  the  glass  and 
is  supposed  to  be  perpendicular  to  the  surface  of  con- 
tact between  the  air  and  the  glass. 

Any  substance  which  retards  the  speed  of  the  wave 
more  than  glass  will  cause  a  greater  change  in  the  direc- 
tion of  the  wave  front,  and,  therefore,  in  the  path  taken 
by  this  portion  of  the  wave. 

When  a  wave  passes  obliquely  from  glass  into  air,  the 
part  of  the  wave  which  enters  the  air  first  moves  faster 
than  the  part  of  the  wave  still  travelling  through  glass. 
The  result  is  that  by  the  time  the  entire  wave  has  entered 
the  air  its  front  makes  a  larger  angle  with  the  surface  of 
the  glass  than  it  did  while  it  was  completely  within  the 
glass. 

The  mathematical  statement  of  this  change  of  direction 
is  that  light  bends  from  the  perpendicular  on  passing  from 
glass  into  air. 

The  direction  in  which  light  travels  is  similarly  changed 
on  passing  from  any  substance  by  which  it  is  more  re- 
tarded, into  another  substance  by  which  it  is  less  retarded, 
the  amount  of  change  in  direction  depending  upon  dif- 
ferences in  the  amount  of  retardation  offered  by  the  two 
substances  concerned. 


250  ELEMENTARY   PHYSICS 

It  is  necessary  to  remember  that  when  light  is  said  to 
pass  through  air,  water,  or  glass,  the  waves  are  in  reality 
passing  through  the  same  substance, — ether.  In  some 
manner,  the  atoms  imbedded  within  the  ether  retard  the 
speed  of  the  waves  passing  through  it,  each  kind  of  sub- 
stance retarding  the  speed  of  these  waves  to  a  different 
degree. 

228.  The  Refraction  of  Light  Passing  Through  a  Prism. 
— When  light  passes  from  air  into  glass,  it  bends  toward 
the  perpendicular.  When  light  passes  from  glass  into 
air,  it  bends  from  the  perpendicular.  If  a  piece  of  glass 
is  so  cut  that  the  surface  by  means  of  which  the  light 
leaves  the  glass  has  a  different  direction  from  that  at 
which  the  light  enters  (§  225),  the  light  after  leaving  the 
glass  will  have  a  different  direction  from  that  at  which  it 
entered.  By  placing  the  second  surface  at  the  proper 
angle,  it  is  possible  to  make  this  surface  bend  tke  light 
still  farther  from  its  original  course  than  it  was  bent  by 
the  first  surface. 

Secure  a  large  glass  prism  with  flat  ends,  so  that,  when 
the  prism  is  set  up  on  one  of  these  ends,  the  prismatic 
faces  will  be  vertical.  A  triangular  piece  of  plate  glass 
with  polished  edges  or  a  bisulphide  of  carbon  prism  will 
do  just  as  well.  Stick  a  pin  into  the  table  several  inches 
behind  the  prism  and  look  at  it  through  the  sides  of  the 
prism.  Slowly  turn  the  prism  until  the  apparent  position 
of  the  pin,  as  seen  through  the  sides  of  the  prism,  is  as 
far  removed  as  possible  from  the  actual  position.  Then, 
without  moving  the  head  or  the  prism,  place  in  contact 
with  the  more  distant  face  of  the  prism  a  second  pin, 
locating  the  point  at  which  the  light  coming  from  the 
pin  to  the  eye  enters  the  glass  (Fig.  107).  In  contact  with 
the  nearer  face  place  a  third  pin  marking  the  point  at 


RADIANT   ENERGY— HEAT   AND   LIGHT     251 

which  the  light  leaves  the  glass.  Between  the  eye  and 
the  prism  place  a  fourth  pin,  hiding  the  third  pin,  to 
determine  in  what  direction  the  light  travels  after  leaving 
the  third  pin.  While  looking  through  the  glass  the  four 
pins  appear  to  be  standing  in  a  straight  line. 

Kemove  the  prism  and  connect  the  pins,  in  order,  with 
straight  lines.  It  will  be  found  that  the  path  of  the 
light  has  twice  changed  its  course,  once  on  entering  the 
glass,  and  again  on  re-entering  the  air.  Both  changes 


J 


FIG.  107. 

result  in  turning  the  light  toward  the  same  side  of  the 
original  path.  Mark  the  positions  of  all  pins,  and  of 
the  two  faces  of  the  prism  through  which  the  light 
passed  on  its  way  from  the  pin  to  the  eye. 

229.  The  Refraction  of  Yellow  Light — Send  a  ray  of  yel- 
low light  through  the  same  prism  in  such  a  manner  that 
it  may  follow  as  nearly  as  possible  the  path  of  light  re- 
corded in  the  preceding  experiment,  and  locate  the  di- 
rections followed  by  the  yellow  light  after  it  has  passed 


252 


ELEMENTARY  PHYSICS 


through  the  prism.     In  order  to  do  this  with  accuracy, 
the  following  apparatus  will  be  found  convenient. 

A  source  of  yellow  light. — When  the  access  of  air  to  the 
lower  part  of  a  Bunsen  burner  is  not  cut  off,  it  burns  with 
a  faint,  bluish  flame  which  is  scarcely  visible.  However, 
if  sodium  carbonate  be  placed  in  the  flame,  it  gives  off  an 
intense  yellow  light.  The  most  convenient  manner  of 
getting  sodium  carbonate  into  the  flame  is  to  dip  one 
end  of  a  platinum  wire  in  hydrochloric  acid;  then  dip 


PIG.  108. 

this  end  into  powdered  sodium  carbonate,  and  hold  it 
in  the  flame  cf  the  burner  (Fig.  108).  The  wire  is  sup- 
plied  with  a  glass  handle  (§  36). 

A  screen  with  a  narrow  slit — to  permit  the  passage  of 
only  a  narrow  beam  of  light.  This  screen  can  be  easily 
made  by  boring  a  hole  an  inch  or  more  in  diameter  through 
a  thin  board  about  8  inches  wide  and  12  inches  long  and 
tacking  two  straight-edged  strips  of  tin  over  this  hole  so 
as  to  form  a  vertical  slit  about  half  as  wide  as  the  thick- 


RADIANT   ENERGY— HEAT  AND   LIGHT     253 

ness  of  a  pin.  The  hole  should  be  bored  in  such  a  posi- 
tion that,  when  the  board  is  set  on  end  on  a  table  and  a 
Bunsen  burner  is  placed  by  the  side  of  it,  the  hole  will 
be  on  a  level  with  the  flame  of  the  burner.  On  each  of 
the  lower  corners  of  the  board  nail  a  small  strip  of  wood 
to  serve  as  a  support  to  keep  the  screen  in  a  vertical 
position. 

Set  up  a  carbon  bisulphide  prism  at  a  distance  of  about 
6  inches  from  the  slit  of  the  screen,  and  place  the  Bunsen 
burner  on  the  other  side  of  the  screen  at  a  distance  of 
about  10  inches  from  the  slit.  The  prism,  slit,  and  lu- 
minous part  of  the  flame  should  have  the  same  elevation 
above  the  table,  so  that  when  in  position  they  lie  in  the 
same  horizontal  plane.  On  looking  at  the  slit  through 
the  prism  we  find  that  the  yellow  light,  resulting  from 
the  use  of  sodium  carbonate,  comes  through  the  slit  and 
is  bent  from  its  straight  path  by  the  prism  in  a  manner 
similar  to  that  discussed  in  connection  with  the  preced- 
ing experiment. 

230.  The  Refraction  of  Red  Light.— Use  lithium  car- 
bonate instead  of  sodium  carbonate.  The  flame  is  col- 
ored strong  red.  The  red  light  is  also  refracted  by  the 
prism,  but  it  is  refracted  less  than  the  yellow  light.  If 
both  sodium  carbonate  and  lithium  carbonate  are  placed 
in  the  flame  at  the  same  time,  the  flame  is  colored  with  a 
mixture  of  yellow  and  red  light.  The  prism,  however, 
will  separate  this  mixture  into  its  elements,  and  the  yel- 
low light  and  the  red  light  are  seen  through  the  prism  as 
two  distinct  narrow  vertical  lines  (Fig.  108). 

If  a  prism  can  separate  all  compound  colors  in  this 
manner,  it  is  possible  to  use  a  slit  and  prism  in  order  to 
determine  what  simple  colors  are  present  in  any  com- 
pound color. 


254 


ELEMENTARY  PHYSICS 


231.  White  is  a  Compound  Color.— The  light  of  the  sun 
is  white.  This  fact  may  be  recognized  readily  when  sun- 
light falls  upon  a  piece  of  paper  on  the  floor. 

Without  shifting  either  the  screen  or  the  prism,  remove 
the  Bunsen  burner  and,  by  the  use  of  one  or  more  mirrors, 
reflect  a  little  sunlight  along  the  path  formerly  followed 
by  the  yellow  and  red  lights  in  their  course  through  the 
slit  toward  the  prism  ;  as  a  result,  a  broad  band  of  light 
formed  by  a  regular  series  of  colors  with  red  at  one  end 
and  violet  at  the  other  is  seen  through  the  prism.  The 


Ultra-red  rays 
(luyisible) 


Red       Or.  Yell.  Green    Blue  .Indigo 
; (Visible  Rays) 


Violet     Ultra-violet  rays 
>     (invisible; 


FIG.  109. 


order  of  occurrence  of  these  colors  is  :  red,  orange,  yel- 
low, green,  blue,  indigo,  and  violet  (Fig.  109). 

If  these  colors  be  examined  more  closely,  it  will  be 
found  that  each  so  called  color  in  reality  consists  of 
many  tints  of  color,  so  that  there  are  many  tints  of  red, 
many  tints  of  green  and  many  tints  of  the  other  colors. 
These  tints  grade  into  each  other  imperceptibly,  so  that 
it  is  impossible  to  find  any  natural  line  of  separation  be- 
tween the  red  and  orange,  orange  and  yellow,  yellow  and 
green,  or  any  other  neighboring  colors  of  this  band. 
Such  lines  ot  separation  are  often  drawn  for  the  sake  of 


RADIANT   ENERGY— HEAT  AND   LIGHT     255 

convenience  in  discussing  the  different  tints  produced, 
but  their  location  is  arbitrary. 

It  is,  therefore,  evident  that  the  white  light  of  the  sun 
consists  of  a  great  many  colors,  or  at  least  of  a  great 
many  tints.  The  number  of  tints  is  so  great  that,  even 
when  they  have  been  separated  by  means  of  a  prism, 
they  form  a  practically  continuous  band  of  color.  While 
it  is  convenient  to  speak  of  this  band  as  consisting  of 
seven  colors,  it  should  be  remembered  that,  in  reality,  it 
consists  of  thousands  of  tints.  The  prism  has  separated 
these  colors  in  such  a  manner  that  they  can  be  studied 
separately. 

White  light  is  a  compound  color.  It  contains  a  greater 
number  of  simple  colors  than  any  other  compound  color 
known.  In  fact,  it  contains  all  of  the  simple  colors.  For 
that  reason,  the  colors  present  in  white  light  are  used  as 
the  standard  with  which  the  simple  colors  found  in  all 
other  compound  colors  are  compared.  Among  the  colors 
present  in  white  light  may  be  located  the  particular  tint 
of  yellow  produced  by  sodium  carbonate,  and  the  partic- 
ular tint  of  red  produced  by  lithium  carbonate.  Since 
the  simple  colors  present  in  white  light  can  be  recognized 
only  when  separated  by  means  of  a  prism,  they  are  called 
the  prismatic  colors,  and  the  band  of  colors  formed  is 
called  the  spectrum  of  white  light.  When  the  sun  is  the 
source  of  the  white  light,  the  spectrum  is  commonly 
spoken  of  as  the  solar  spectrum. 

232.  The  Growth  of  the  Spectrum  During  Increase  of 
Temperature  on  Heating  a  Platinum  Ball, — Place  a  Bun- 
sen  burner  in  such  a  position  that  when  a  platinum  ball, 
held  in  the  flame,  becomes  hot  enough  to  emit  light,  the 
light  will  pass,  through  the  slit  and  the  prism,  toward 
the  eye,  as  in  the  preceding  experiments.  The  first 


\  8  R  A 
THE 

I  /  M  I  ./  me 


256  ELEMENTARY   PHYSICS 

light  which  the  ball  gives  out  on  being-  heated  has  a 
dull  red  color.  In  tint  it  corresponds  to  the  dull  red  at 
the  extreme  limit  of  one  end  of  the  solar  spectrum. 
Looked  at  through  the  prism  this  dull  red  color  is  seen 
as  a  vertical  line.  As  the  heating  of  the  ball  continues, 
its  color  not  only  becomes  more  intensely  red  but  it  also 
changes  in  tint.  If  during  this  change  of  color  the  light 
coming  from  the  ball  is  examined  through  the  prism,  it 
is  noticed  that  new  tints  of  red  are  added  to  those 
already  formed  so  that  the  original  vertical  line  of  dull 
red  appears  to  broaden  out  into  a  band  of  red.  Since 
the  growth  of  the  band  takes  place  in  one  direction  only, 
the  dull  red  color  first  seen  always  forms  one  end  of  the 
band,  while  the  new  tints  of  red  are  added  in  succession 
to  the  other  end. 

The  addition  of  new  tints  of  red  is  only  one  of  the 
changes  to  be  noticed  on  heating  the  ball.  If  the  tints 
of  red  successively  added  to  those  already  present  be  ex- 
amined closely,  it  will  be  seen  that  they  become  brighter, 
or  more  intense.  The  examination  of  the  ball  through 
the  prism,  therefore,  shows  that  as  the  ball  grows  hotter, 
new  tints  of  red  are  added  and  that  those  already  pres- 
ent become  more  intense. 

The  brighter  red  color  of  the  platinum  ball  as  seen 
without  the  prism  is  evidently  due  to  the  combined  action 
of  all  of  the  changes  just  described,  but  without  the  use  of 
the  prism  it  is  not  likely  that  the  exact  nature  of  the 
changes  of  color  of  the  ball  would  ever  have  been  detected. 
The  prism  makes  it  possible  to  determine  to  what  the 
changes  in  the  color  of  the  ball  are  due. 

As  the  heating  of  the  ball  is  continued,  the  band  seen 
through  the  prism  continues  to  grow — by  adding  vari- 
ous tints  of  orange-red,  then  of  reddish-orange,  orange- 


RADIANT   ENERGY;  HEAT   AND   LIGHT      257 

yellow,  yellow,  yellowish-green,  green,  greenish-blue, 
blue,  bluish-violet  and  violet,  always  to  the  same  side  of 
the  band,  until  all  of  the  colors  seen  in  the  solar  spectrum 
are  present.  In  the  meantime  the  intensity  of  the  tints 
first  present  increases  as  before. 

The  changes  in  the  compound  colors  produced  by  the 
addition  of  new  tints  as  the  heating  progresses  are  far 
less  striking.  First  there  is  a  mixture  of  the  different 
red  tints ;  then  a  mixture  of  red  and  orange,  producing  a 
bright  red  effect ;  then  a  mixture  of  red,  orange,  and 
yellow,  which  gives  a  somewhat  yellowish  tinge  to  the 
red.  As  the  other  colors  of  the  spectrum  are  added  to 
those  already  present,  the  compound  color  varies  from 
reddish-yellow  to  white.  Since  the  red  tints  of  the  spec- 
trum are  the  first  to  appear  when  a  platinum  ball  is  heated, 
these  are  the  first  colors  to  become  very  intense.  There- 
fore, red  strongly  predominates  in  the  compound  colors 
produced  during  the  earlier  part  of  the  heating  of  the  ball. 

233.  Each  Prismatic  Color  in  the  Spectrum  is  Due  to  a 
Different  Rate  of  Vibration. — The  original  cause  of  sound 
is  the  vibration  of  some  body.  This  vibratory  motion  is 
taken  up  by  the  air  and  is  transmitted  in  the  form  of 
waves.  Increase  in  the  rate  of  vibration  of  the  sounding 
body  results  in  an  increase  in  the  number  of  waves  sent 
out  per  second.  This  is  recognized  by  the  ear  as  an  in- 
crease in  pitch.  Increase  in  the  amplitude  of  vibration 
of  the  sounding  body  causes  stronger  condensations  in 
the  air.  In  consequence  the  waves  move  forward,  not 
more  swiftly,  but  with  greater  energy.  Hence,  the  sound 
heard  is  louder.  Anything  which  causes  a  greater  quan- 
tity of  air  to  be  set  in  motion,  for  instance,  a  sounding- 
board,  also  increases  the  total  energy  with  which  the 
waves  move  forward,  and  adds  to  the  loudness  of  sound. 


258  ELEMENTARY  PHYSICS 

In  the  same  manner,  the  original  cause  of  light  is  the 
vibration  of  atoms,  or  of  the  combinations  of  atoms  called 
molecules.  This  vibratory  motion  is  taken  up  by  the 
ether  and  is  transmitted  in  the  form  of  waves.  Increase 
in  the  rate  of  vibration  of  the  atoms  results  in  an  increase 
in  the  number  of  waves  sent  forward  by  the  ether  in  each 
second.  If  these  vibrations  are  sufficiently  rapid,  the 
waves  produce  the  sensation  of  light  and  color.  Any 
further  increase  in  the  rate  of  vibration  of  the  atoms 
results  in  a  change  of  color,  the  tint  produced  being  nearer 
the  violet  end  of  the  solar  spectrum.  Increase  in  the 
amplitude  of  vibration  of  the  atoms  causes  the  waves  set 
up  in  the  ether  to  move  with  greater  energy.  This  is 
recognized  as  an  increase  in  the  intensity  of  the  color 
produced. 

Most  sounding  bodies  produce  simultaneously  many 
vibrations  differing  in  intensity  and  rate  of  vibration. 
This  has  already  been  discussed  in  connection  with  funda- 
mental tones  and  overtones.  The  waves  set  up  simulta- 
neously in  the  air  produce  a  combined  effect,  recognized 
as  the  characteristic  quality  of  the  sound. 

In  a  like  manner,  the  various  atoms  of  the  same  heated 
body  may  differ  so  much  in  their  rate  and  intensity  of 
vibrations  that  only  the  combined  effect  can  be  recognized 
without  the  use  of  a  prism.  This  combined  effect  may 
also  result  in  the  perception  of  color,  but  it  is  a  com- 
pound color. 

Something  similar  to  this  is  unquestionably  true  also 
in  the  case  of  heat,  since  heat  and  light  are  due  to  the 
same  phenomena.  The  phenomena  of  heat,  however,  are 
usually  investigated  in  an  entirely  different  manner  from 
the  phenomena  of  light,  and  we  have  no  terms  which  are 
strictly  comparable  with  the  terms  pitch  and  loudness  in 


RADIANT   ENERGY,   HEAT   AND   LIGHT     259 

sound,  or  with  tint  and  intensity  as  applied  to  the  spec- 
trum colors.  The  temperature  of  a  body  depends,  of 
course,  on  the  rate  of  vibration  of  its  atoms  or  of  the 
groups  of  atoms  known  as  molecules,  but  the  temperature 
of  a  body  as  registered  by  a  thermometer  depends  also 
upon  the  amplitude  of  vibration  of  the  molecules  and  upon 
the  number  of  the  molecules  which  are  vibrating ;  in  other 
words,  upon  the  size  of  the  body.  The  quantity  of  heat 
in  a  body  depends  also  upon  the  rate  and  amplitude  of 
vibration  of  its  molecules  and  upon  the  number  of  mole- 
cules present  in  the  body.  However,  a  great  quantity  of 
heat  usually  implies  a  large  body,  while  a  high  tempera- 
ture may  be  shown  by  a  small  body. 

234.  Vibrations  whose  Rates  are  too  Slow  to  Produce 
Light  are  Deflected  Beyond  the  Red  End  of  the  Spectrum. 
— All  waves  pass  through  the  ether  at  the  same  speed. 
The  waves  due  to  the  less  rapidly  vibrating  atoms  which 
produce  a  red  color,  pass  through  the  ether  with  the  same 
rapidity  as  the  waves  due  to  vibrations  sufficiently  rapid 
to  produce  a  violet  color.  Since  each  vibration  of  an 
atom  gives  rise  to  one  wave,  a  greater  number  of  waves 
must  be  produced  in  one  second  by  a  rapidly  vibrating 
atom  than  by  one  vibrating  more  slowly.  If  the  rate  of 
vibration  of  an  atom  is  regular,  it  will  send  off  waves  at 
equal  intervals  of  time.  These  waves  must,  therefore,  be 
an  equal  distance  apart.  If  the  distance  from  one  wave 
to  the  next  is  called  a  wave  length,  the  length  of  the 
waves  produced  by  a  rapidly  vibrating  atom  must  be 
shorter  than  the  length  of  the  waves  produced  by  a  more 
slowly  vibrating  atom.  Hence  the  waves  producing  the 
colors  nearer  the  violet  end  of  the  spectrum  must  be 
shorter  than  those  producing  the  colors  nearer  the  red 
end.  An  examination  of  the  solar  spectrum  shows  that 


260  ELEMENTAKY   PHYSICS 

the  larger  waves  producing  colors  near  the  red  end  of  the 
spectrum  are  deflected  less  than  the  shorter  waves. 

Vibrations  of  atoms  too  slow  to  give  rise  to  the  sensa- 
tion of  light  also  produce  waves.  These  waves  must  be 
longer  even  than  those  of  the  dull  red  at  one  end  of 
the  spectrum.  Although  invisible,  they  also  must  pass 
through  the  prism,  but  should  be  deflected  less  and  hence 
the  search  for  them  should  be  conducted  beyond  the  red 
end  of  the  spectrum.  How  may  these  waves  be  detected 
if  they  are  not  capable  of  producing  the  sensation  of 
light  ?  Some  instrument  other  than  the  eye  must  be  used. 
A  very  delicate  thermometer  might  be  serviceable.  A 
thermopile,  however,  is  better. 

235.  A  Thermopile   is   Used  to  Detect  Extremely  Small 
Variations  in  Temperature. — A  thermopile  is  an  electrical 
instrument  which  is  capable  of  detecting  much  smaller 
changes  in  temperature  than  even  the  most  delicate  ther- 
mometer.   Thermopiles  have  been  constructed  which  are 
capable  of  detecting  a  variation  in  temperature  of  less  than 
one-millionth  of  a  degree.     It  is  not  necessary  to  describe 
the  instrument  here,  since  a  full  description  may  be  found 
in  any  larger  book  on  physics.     A  slit  thermopile  is  most 
convenient  in  detecting  the  presence  of  waves  which  have 
been  deflected  by  a  prism.    The  method  of  its  use  is  suffi- 
ciently well  explained  in  "A  School  Course  in  Heat,"  by 
Larder,  Sampson  Low,  Marston  and  Co.  London. 

236.  Relative  Intensity  of  Heat  Produced  by  the  Different 
Waves  Giving  Rise  to  the  Solar  Spectrum. — The  use  of  the 
thermopile  may  be  illustrated  by  investigating  the  rela- 
tive intensity  of  heat  produced  by  the  waves  giving  rise 
to  the  solar  spectrum.     Cause  a  small  quantity  of  light 
from  the  sun  to  pass  through  a  narrow  slit  and  a  prism, 
and  then  allow  it  to  strike  the  surface  of  a  large  piece  of 


RADIANT  ENERGY,   HEAT  AND   LIGHT     261 

white  paper.  The  prism  will  deflect  the  different  tints  to 
different  positions  on  the  paper,  and  the  result  will  be  the 
appearance  on  its  surface  of  a  broad  band  of  colors,  the 
solar  spectrum. 

By  moving  the  thermopile  in  front  of  the  paper  so  as  to 
permit  the  various  tints  to  fall  in  succession  upon  the 
thermopile,  the  relative  intensity  of  heat  given  out 
by  the  different  waves  producing1  light  can  be  compared. 
Instead  of  a  glass  prism,  a  salt  prism  is  used,  since  salt 
absorbs  far  less  of  the  wave  motion  than  glass.  §  255. 

By  studies  of  this  kind  it  has  been  discovered  that  the 
lowest  intensity  of  heat  is  produced  by  the  short  waves  at 
the  violet  end  of  the  spectrum,  and  that  the  intensity  of 
heat  increases  on  approaching  the  red  end  of  the  spec- 
trum (Fig.  109).  A  similar  fact  is  true  of  the  spectrum  of 
the  platinum  ball. 

The  greater  intensity  of  heat  produced  by  the  longer 
waves  due  to  the  lower  rates  of  vibration  in  the  case  of  the 
platinum  ball  is  possibly  due,  not  only  to  the  greater  in- 
tensity of  these  vibrations,  but  also  to  the  fact  that,  even 
when  the  platinum  ball  has  become  white  hot,  the  number 
of  molecules  vibrating  at  a  lower  rate  is  probably  still 
much  greater  than  the  number  of  molecules  vibrating  at 
a  higher  rate.  The  same  supposition  may  also  be  made 
to  account  for  the  similar  phenomena  just  described  in 
the  case  of  the  spectrum  of  the  sun. 

X,  237.  Relative  Intensity  of  Heat  Produced  by  the  Invisible 
Waves  Deflected  Beyond  the  Red  and  the  Violet  Ends  of  the 
Spectrum. — It  has  been  shown  that,  when  a  thermopile  is 
moved  from  the  violet  to  the  red  end  of  the  spectrum,  an 
increase  of  temperature  is  indicated.  If  the  motion  of 
the  thermopile  is  continued  in  the  same  direction,  be- 
yond the  red  end  of  the  spectrum,  it  is  found  that  the 


262  ELEMENTARY  PHYSICS 

thermopile  indicates  an  increase  in  intensity  of  heat 
even  after  the  dull  red  tint  at  the  very  extreme  of  the 
visible  part  of  the  spectrum  has  been  passed  (Fig.  109). 
This  increase  continues  until  a  certain  distance  beyond 
the  end  of  the  visible  spectrum  has  been  reached,  after 
which  the  thermopile  indicates  a  decrease  in  the  intensity 
of  heat.  The  fact  that  the  thermopile  is  affected  at  all 
indicates  the  presence  of  waves  beyond  the  red  end  of 
the  spectrum  and  that  these  waves  may  be  detected  by  the 
thermopile  even  when  they  cannot  be  seen  by  the  eye. 

It  is  an  interesting  fact  that  the  greatest  intensity  of 
heat  is  produced,  not  by  the  waves  causing  the  red  colors 
of  the  spectrum,  but  by  invisible  waves  having  a  consider- 
ably greater  length,  whose  position  is  some  distance  be- 
yond the  red  end  of  the  spectrum. 

The  possibility  of  invisible  waves  producing  great 
intensity  of  heat  is  shown  by  the  name  of  the  Bunsen 
burner,  which  is  intensely  hot,  but  which  (when  well 
regulated)  scarcely  gives  out  any  light. 

If  the  thermopile  is  now  moved  in  the  opposite  direc- 
tion, from  the  red  to  the  violet  end  of  the  spectrum,  it  is 
found  that  invisible  waves  exist  also  in  the  region  be- 
yond the  violet  end  of  the  spectrum  (Fig.  109).  These 
waves  are  shorter  than  any  visible  in  the  spectrum,  but 
they  are  unable  to  produce  the  sensation  of  light.  More- 
over they  cause  even  a  less  intensity  of  heat  than  the  vio- 
let rays.  The  farther  the  waves  are  beyond  the  violet  end 
of  the  spectrum,  the  less  is  the  intensity  of  heat. 

238.  Ultra-Red  and  Ultra- Violet  Parts  of  the  Spectrum. 
— The  word  spectrum  was  originally  used  to  designate 
the  band  of  colors  produced  by  light  passing  through  a 
prism.  Since  the  prism  separates  all  waves  passing 
through  it,  both  visible  and  invisible,  and  distributes  them 


KADIANT   ENERGY,    HEAT  AND   LIGHT     263 

in  the  order  of  their  length,  it  is  possible  to  speak  of 
the  entire  series  of  waves  after  they  leave  the  prism,  as 
the  spectrum.  That  part  of  this  spectrum  which  includes 
the  waves  capable  of  producing  the  sensation  of  light  is 
then  called  the  visible  spectrum.  The  invisible  waves 
which  are  deflected  to  points  beyond  the  red  end  of  the 
spectrum  form  the  ultra-red  spectrum,  and  the  invisible 
waves  which  are  deflected  beyond  the  violet  end  of  the 
spectrum  form  the  ultra-violet  spectrum. 

239.  Nerves  of  Sight  Not  Sensitive  to  All  Rates  of  Vibra- 
tion.— The  vibrations  which  produce  the  dull  red  tint  at 
the  red  end  of  the  visible  spectrum  have  a  rate  of  392  trill- 
ions per  second ;  the  vibrations  at  the  extreme  limit  of  the 
violet  end  of  the  visible  spectrum  have  a  rate  of  752  trill- 
ions per  second.    Some  eyes  are  sensitive  to  lower  and  some 
to  higher  rates  of  vibrations  than  others.     This  merely 
corroborates  the  view,  that  the  inability  of  the  eye  to  see 
all  vibrations  transmitted  through  the  ether  is  due  not  to 
any  special  difference  between  visible  and  invisible  vibra- 
tions, aside  from  their  rate  of  vibration,  but  is  caused  by 
a  lack  of  sufficient  sensitiveness  in  the  nerves  of  the  eye. 

As  already  stated  there  are  vibrations  whose  rate  is 
less  than  that  giving  rise  to  red  at  the  extreme  end  of  the 
visible  end  of  the  spectrum.  In  addition  to  the  rays 
which  are  capable  of  giving  rise  to  the  sensation  of  heat, 
but  not  of  light,  are  found  waves  whose  frequency  is  very 
much  less.  Some  of  these  are  known  as  the  Hertz  waves, 
and  produce  the  results  utilized  in  wireless  telegraphy. 
On  the  other  hand,  the  frequency  of  the  vibrations  present 
in  X-rays  very  much  exceeds  the  frequency  shown  by  the 
violet  rays  at  the  other  end  of  the  spectrum. 

240.  Natural  and  Artificial  Sources  of  Light. — In  order 
that  light  may  be  seen,  there  must  be  a  source  of  light,  a 


264  ELEMENTARY   PHYSICS 

source  capable  of  producing  those  waves  to  which  the 
eye  is  sensitive.  Very  few  bodies  act  as  sources  of  light 
or  are  self-luminous.  Most  bodies  merely  reflect  or 
transmit  light.  The  sun  is  the  most  important  source  of 
light.  The  moon  merely  reflects  a  part  of  the  sunlight 
which  falls  upon  it.  That  part  of  the  moon's  surface 
upon  which  the  light  of  the  sun  does  not  fall  remains 
dark.  The  stars  are  also  sources  of  light,  as  may  be  seen 
easily  on  a  night  when  the  moon  is  not  shining.  The 
most  important  artificial  sources  of  light  are  those  due  to 
rapid  combustion :  such  as,  the  burning  of  wood,  coal, 
coal-oil,  acetylene  gas,  and  other  materials.  The  light 
given  out  by  the  flame  is  due  to  particles  of  carbon  which 
once  were  parts  of  the  wood,  or  coal-oil,  and  which  for  a 
moment  are  heated  enough  to  give  out  light.  Any  parti- 
cles not  burned  up  before  they  leave  the  flame  remain- un- 
consumed  and  pass  off  as  smoke.  In  the  incandescent 
electric  light  the  carbon  filament  is  heated  white  hot,  but 
is  not  burnt. 

The  flash  of  lightning  is  due  to  intense,  rapid  vibra- 
tions of  the  atoms  forming  the  air,  set  in  motion  by  elec- 
tricity. The  light  given  out  by  the  fire-fly  is  also  due  to 
very  rapid  vibrations  of  atoms.  The  heat  produced  by  the 
fire-fly  is  small.  This  may  be  due  to  the  small  intensity 
of  the  vibrations  belonging  to  the  red  and  ultra-red  parts 
of  the  spectrum,  or  to  the  small  number  of  vibrating  atoms. 

241.  Different  Sources  of  Light  Give  Out  Different  Colors. 
— Different  sources  of  light  may  produce  different  waves, 
or  may  produce  waves  which  are  alike  in  length  but  dif- 
fer in  energy.  It  has  already  been  shown  that  sodium 
carbonate,  placed  in  the  flame  of  a  Bunsen  burner,  gives 
out  yellow  light,  and  that  lithium  carbonate  gives  out 
red  light.  In  reality  the  compounds  are  broken  np,  and 


RADIANT   ENERGY,    HEAT   AND   LIGHT     265 

the  elements,  sodium  and  lithium,  give  out  the  yellow 
and  red  light.A  Other  substances  give  out  different  colors. 
The  light  from  a  coal-oil  lamp  consists  of  all  the  waves 
present  in  sunlight,  but  the  yellow  and  red  colors  are 
relatively  more  intense  and  the  blue,  bluish-violet,  and 
violet  colors  less  intense,  than  corresponding  colors  in 
the  light  from  the  sun.  Hence  objects  illuminated  by 
lamplight  often  appear  to  have  colors  differing  from  the 
colors  of  the  same  objects  seen  in  daylight. 

242.  Most  Objects  are  Visible  Only  While  they  Reflect 
or  Transmit  Light. — Most  objects  do  not  give  out  light  of 
their  own.     They  are  visible  only  by  means  of  the  light 
which    they  receive  from  various  sources  of  light,  and 
then  either  reflect  or  transmit. 

If  a  room  were  closed  so  that  not  even  a  single  ray  of 
light  could  enter  it,  and  if  no  artificial  source  of  light  (a 
lamp  or  electric  light)  were  introduced,  not  a  single  object 
in  the  room  could  be  seen.  During  nights  when  the 
moon  is  not  shining,  and  when  the  clouds  are  thick  enough 
to  absorb  almost  all  of  the  light  from  the  stars,  we  are 
able  to  appreciate  more  fully  the  fact,  that  most  objects 
can  be  seen  only  because  they  reflect  or  transmit  light. 

Even  a  tiny  ray  of  light  illuminates  many  parts  of  a 
darkened  room.  The  light  is  reflected  from  object  to  ob- 
ject, and  scattered  in  so  many  directions  by  any  uneven 
surface,  that  a  little  light  is  finally  reflected  from  most 
objects  in  the  room.  In  a  similar  manner,  objects  not  in 
direct  sunlight  receive  the  light  of  the  sun  after  reflec- 
tion from  many  other  bodies. 

243.  Difference  of  Color  of  Opaque  Objects  Due  to  their 
Inability  to   Reflect   All  or  a   Part  of  Certain  Prismatic 
Colors. — All  objects  which  are  not  transparent  are  visible 
only  when  they  reflect  light.     The  light  which  they  re- 


266  ELEMENTARY   PHYSICS 

fleet  depends  up6n  the  light  which  falls  upon  them.  It 
is  a  familiar  fact  that,  when  red  light  alone  falls  upon  a 
piece  of  white  paper,  the  paper  appears  red.  When  yel- 
low light  alone  falls  upon  it,  the  paper  appears  yellow. 
But  this  is  not  true  of  all  objects.  All  objects  cannot 
reflect  the  same  color.  Therefore  the  color  of  an  opaque 
object  depends,  not  only  upon  the  colors  which  fall  upon 
it  but  also  upon  the  colors  which  it  can  reflect. 

This  may  be  shown  by  taking  some  source  of  light 
which  contains  all  of  the  prismatic  colors,  and  investigat- 
ing which  of  these  colors  different  bodies  can  reflect. 

By  means  of  a  mirror  reflect  a  beam  of  sunlight  through 
a  narrow  slit  into  a  darkened  room.  Let  the  light  pass 
through  a  prism,  as  before,  and  then  let  it  fall  upon  a 
sheet  of  white  paper.  The  paper  is  white,  because  it  re- 
flects all  the  colors  present  in  sunlight.  It  is  able,  there- 
fore, to  reflect  each  one  of  these  colors  singly,  after  all 
the  colors  have  been  separated  by  the  prism.  This  is  the 
reason  for  the  use  of  white  paper  in  this  experiment. 
Change  the  position  of  the  paper,  until  the  prismatic 
colors  spread  over  as  large  an  area  as  possible. 

Place  a  piece  of  red  flannel  in  front  of  the  different 
colors  of  the  spectrum.  When  red  light  falls  upon  it,  it 
has  its  usual  strong  red  appearance.  When  other  colors 
fall  upon  it,  it  appears  dark  or  black.  Red  is  the  only 
color  which  it  reflects  well.  Therefore,  it  appears  red  even 
if  all  the  prismatic  colors  present  in  sunlight  fall  upon  it. 
But  when  the  light  which  falls  upon  the  flannel  does 
not  contain  any  red  light,  when  for  instance  it  contains 
only  green  light,  the  flannel  is  not  able  to  reflect  the  green 
light,  the  only  color  which  it  receives.  Since  it  fails 
to  reflect  any  color,  it  appears  black. 

Cover  a  smooth  piece  of  wood  with  soot  by  means  of  a 


RADIANT   ENERGY,   HEAT  AND   LIGHT     267 

lamp-  or  gas-flame.  Hold  it  in  various  parts  of  the  spec- 
trum. It  reflects  none  of  the  prismatic  colors.  Black, 
therefore,  is  the  absence  of  all  color.  Absolute  blackness 
is  due  either  to  a  complete  failure  to  reflect  light,  or  to  a 
complete  absence  of  light  to  be  reflected.  Most  objects 
called  black  are  not  absolutely  black,  but  reflect  a  suffi- 
cient quantity  of  some  one  prismatic  color  to  make  it 
more  appropriate  to  speak  of  the  color  as  bluish-black, 
greenish-black,  or  some  other  shade  of  black. 

244.  Compound  Colors  Due  to  Selective  Reflection.— If  now 
a  great  variety  of  substances  be  tested  by  holding  them 
in  different  parts  of  the  spectrum,  it  will  be  found  that 
some  substances  reflect  only  one  color  well,  and,  therefore, 
possess  a  color  which  approaches  one  of  the  prismatic 
colors.  Other  substances  reflect  one  prismatic  color  well 
and  two  or  three  other  prismatic  colors  in  a  very  moder- 
ate degree.  The  prismatic  color  which  is  well  reflected 
usually  gives  its  name  to  the  resulting  compound  color. 
The  other  prismatic  colors  reflected  determine  the  char- 
acteristic tint.  The  general  color  effect  produced  by  the 
mixture  is  usually  very  different  from  the  effect  of  any  of 
the  prismatic  colors  :  for  example,  greenish-blue,  orange- 
red. 

Many  substances  reflect  a  number  of  prismatic  colors 
in  such  quantities  that  the  compound  color  produced 
has  no  resemblance  to  any  of  the  prismatic  colors.  By 
persons  not  students  they  are  often  looked  upon  as  of 
equal  rank  with  prismatic  colors.  Brown,  gray,  drab, 
lilac  are  compound  colors. 

Among  the  objects  to  be  examined  it  is  well  to  have 
quite  a  number  which  are  distinctly  and  strongly  colored. 
Light-colored  objects  appear  light,  because  they  reflect 
a  considerable  amount  of  almost  all  of  the  other  prismatic 


268  ELEMENTARY   PHYSICS 

colors,  in  addition  to  the  prismatic  colors  which  strongly 
predominate  ;  for  example,  light-red  ribbons. 

245.  Reflection  of  Heat. — Since  most  substances  are  not 
able  to  reflect  all  of  the  prismatic  colors  which  form  the 
visible  part  of  the  spectrum,  it  is  probable  that  most  sub- 
stances also  cannot  reflect  all  of  the  waves  which  form  the 
invisible  part  of  the  spectrum.     Moreover,  since  both  vis- 
ible and  invisible  waves  can  produce  the  sensation  of  heat, 
this  is  practically  equivalent  to  saying  that   most  sub- 
stances vary  considerably  in  their  ability  to  reflect  heat. 
This  subject  is  difficult  to  investigate,  since  most  of  the 
waves  producing  heat  are  invisible,  but  it  is  well  known 
that  some  bodies  reflect  a  much  greater  portion  of  the 
total  quantity  of  heat  falling  upon  them  than  others. 
Polished  metals  are  among  the  best  reflectors  of  heat. 
Lampblack  is  the  poorest  reflector  of  heat  known. 

246.  Heat  May  be  Reflected  by  a   Cold  Surface.— If  we 
stand  at  one  side  of  an  open  fireplace,  out  of  reach  of  the 
direct  heat,  in  a  room  that  has  not  yet  been  heated  for  a 
sufficient  length  of  time  to  become  warm,  and,  if  some  one 
standing  in  front  of  the  fire  suddenly  places  a  large  bright 
sheet  of  tin  in  such  a  position  that  the  light  of  the  fire  is 
reflected  upon  us,  we  not  only  instantly  perceive  the  light, 
but  we  are  at  the  same  moment  aware  of  a  considerable 
quantity  of  heat.     However,  if  we  touch  the  tin  quickly 
we  find  that  it  is  still  cold.     The  heat  that  has  come  to  us 
is  not  the  heat  of  the  tin,  but  the  heat  of  the  fire  reflected 
by  the  tin. 

247.  Colorless  Transparent  Substances  Transmit  All  of  the 
Prismatic  Colors. — Many  transparent  substances  show  no 
indication  of  color.     If  one  of  these  substances  be  held 
anywhere  along  the  path  of  light  which  is  reflected  by  a 
mirror  or  porte-lumiere  into  a  darkened  room  and  toward 


EADIANT   ENERGY,    HEAT   AND   LIGHT     269 

a  prism,  it  will  be  found  that  the  transparent  substance 
has  absorbed  so  little  of  the  various  prismatic  colors  pres- 
ent in  sunlight,  that  the  spectrum  on  the  screen  remains 
practically  unchanged.  In  fact,  these  substances  are 
called  colorless,  because  they  transmit  all  colors  un- 
changed and,  therefore,  produce  no  perceptible  effect 
upon  the  colors  of  objects  seen  through  them. 

248.  Differences  of  Color  Transmitted  by  Transparent 
Objects  Due  to  Their  Inability  to  Transmit  All  or  a  Part 
of  Certain  Prismatic  Colors.— Any  object  seen  through  a 
piece  of  red  glass  looks  either  red  or  black.  The  reason 
for  this  may  be  readily  seen  if  the  red  glass  is  held  so 
that  the  sunlight  passes  through  the  glass  before  it  enters 
the  prism.  The  spectrum  on  the  screen  is  changed  in  ap- 
pearance. Most  of  the  colors  have  evidently  been  stopped 
by  the  red  glass.  Only  the  red  colors  have  been  allowed 
to  pass  through  freely.  The  other  colors  which  are  trans- 
mitted, are  transmitted  only  in  small  quantities.  If  a 
kind  of  glass  could  be  found  which  would  transmit  abso- 
lutely nothing  but  red,  only  objects  which  give  out,  re- 
flect, or  transmit  red  light,  could  be  seen  through  it. 
Since  only  red  light  would  be  transmitted  by  this  kind  of 
glass,  all  objects  seen  through  the  glass  would  appear  red. 
Objects  not  giving  out  red  light,  but  giving  out  only  other 
colors,  would  be  invisible  and  the  spaces  occupied  by 
them  would  appear  black. 

This  does  not  mean  that  only  those  objects  which  ap- 
pear red  to  the  unassisted  eye  could  be  seen  through  a 
piece  of  red  glass.  It  should  be  remembered  that  most 
of  the  colors  occurring  in  nature  are  compound  colors. 
Most  of  these  colors  contain  (in  addition  to  other  and 
often  more  evident  colors)  more  or  less  of  the  prismatic 
colors  called  red.  For  instance,  the  color  of  an  ob- 


270  ELEMENTARY   PHYSICS 

ject  which  appears  blue  to  the  naked  eye  may  also  contain 
a  small  quantity  of  red,  so  that  when  the  object  is  viewed- 
through  red  glass,  it  will  be  visible,  and  will  apparently 
have  a  dull  red  color.  How  prevalent  these  red  pris- 
matic colors  are  in  almost  all  of  the  combinations  of 
colors  reflected  by  objects  in  nature,  may  be  better  ap- 
preciated by  looking  at  these  objects  through  a  piece  of 
red  glass.  It  will  at  once  become  evident  that  almost 
all  objects  reflect  or  transmit  some  reel  color,  although 
the  quantity  of  red  color  thus  reflected  or  transmitted, 
may  be  so  small  that  it  cannot  be  noticed  until  the  red 
glass  is  used,  or  until  the  prism  separates  the  red  from 
the  other  colors. 

Hold  other  kinds  of  colored  glass  in  the  path  of  the 
sunlight,  before  it  enters  the  prism.  It  will  be  found 
that  green  glass  transmits  chiefly  green  ;  blue  glass, 
chiefly  blue  ;  other  kinds  of  glass,  other  colors.  In  each 
of  these  cases,  the  predominant  prismatic  color  trans- 
mitted gives  name  to  the  color  of  the  glass. 

When  several  prismatic  colors  are  transmitted  in  con- 
siderable quantities,  their  combined  effect  is  often  so  dis- 
similar to  the  effect  of  any  single  prismatic  color,  that 
special  names  have  been  coined  to  designate  the  com- 
bination of  color  thus  produced. 

Throw  a  spectrum  on  the  wall  and  look  at  it  through 
differently  colored  pieces  of  glass. 

249.  Color  of  Glass  Usually  Recognized  by  Light  Reflected 
from  Other  Bodies. — It  is  not  necessary  to  hold  the  glass 
between  the  eye  and  some  source  of  light  in  order  to 
determine  its  color.  No  matter  in  what  position  the 
eye  is  held,  a  sufficient  variety  of  colors  is  usually  re- 
flected by  various  objects  beyond  the  glass  to  enable  the 
glass  to  transmit  to  the  eye  about  the  same  combination 


RADIANT   ENERGY,    HEAT   AND   LIGHT      271 

of  colors  which  it  transmits  when  it  is  exposed  to  sun- 
light. Therefore,  if  the  glass  be  held  between  the  eye 
and  the  floor,  the  eye  and  a  building,  or  the  eye  and  a 
plot  of  grass,  a  range  of  prismatic  colors  will  be  sent  by 
the  various  objects  through  the  glass  to  the  eye  which 
will  be  sufficient  to  give  the  glass  its  usual  color. 

250.  Color  of  Light  Transmitted  Through  a  Succession  of 
Differently  Colored  Substances — If  a  kind  of  glass  could  be 
found  which  transmitted  nothing  but  red,  and  another 
kind  which  transmitted  nothing  but  green,  no  light  could 
be  transmitted  through  both  glasses  if  they  were  held  to- 
gether. The  red  glass  would  transmit  only  the  red  light, 
and  this  red  light  would  be  absorbed  soon  after  it  entered 
the  green  glass,  so  that  nothing  could  pass  through  both. 

As  a  matter  of  fact,  however,  most  green  glass  trans- 
mits not  only  green  light,  but  also  a  little  red  light,  so 
that  objects  seen  through  both  red  and  green  glass  appear 
dull  red.  In  the  same  manner  most  blue  glass  transmits 
not  only  blue  light,  but  also  a  little  red  light,  so  that  ob- 
jects seen  through  both  red  and  blue  glass  appear  dull 
red.  Blue  glass  transmits  a  little  green  in  addition  to 
blue,  and  green  glass  transmits  a  little  blue  in  addition 
to  green,  so  that  objects  seen  through  both  have  a  peculiar 
bluish-green  color  affected  a  little  by  the  red  which  is 
transmitted  by  both  kinds  of  glass.  But  a  combination 
of  red,  green,  and  blue  glasses  will  absorb  so  much  of  all 
the  different  colors,  that  very  little  light  of  any  kind,  ex- 
cept a  faint  tinge  of  red,  can  pass  through  all  of  them. 
The  result  is  that  no  objects  can  be  seen  through  a  com- 
bination of  all  of  the  three  kinds  of  glass  here  mentioned, 
unless  they  give  out  a  very  considerable  quantity  of  red 
light.  Ordinarily  objects  do  not  do  this,  but  the  light 
of  the  sun  or  of  an  electric  light  contains  enough  of 


272  ELEMENTAEY   PHYSICS 

the  strong-  red  colors  to  be  seen  through  all  of  these 
glasses  used  tog-ether. 

251. — Effect  of  Thickness  upon  the  Color  of  Transparent 
Substances. — The  colors  which  enter  glass,  but  which  are 
not  transmitted,  are  absorbed.  They  are  not  absorbed  at 
once,  but  are  absorbed  more  and  more  as  they  penetrate 
deeper  into  the  glass.  The  thickness  of  the  glass  will, 
therefore,  determine  how  much  of  these  colors  will  be 
absorbed.  If  a  thin  layer  of  colored  glass  examined  by 
means  of  a  prism  shows  that  it  absorbs  very  much  more  of 
some  colors  than  of  others,  it  is  certain  that  thicker  layers 
may  absorb  all  of  the  more  readily  absorbed  colors,  while 
they  will  still  transmit  considerable  quantities  of  those 
less  readily  absorbed.  The  compound  colors  produced 
by  light  passing  through  thicker  layers  of  this  kind  of 
glass  will  not  contain  some  of  the  prismatic  colors  which 
are  transmitted  through  thinner  layers.  The  colors  of 
certain  kinds  of  glass,  therefore,  depend,  to  a  certain  ex- 
tent, upon  the  thickness  of  the  glass. 

The  influence  of  thickness  upon  the  color  of  transparent 
substances  may  be  seen  by  placing  an  increasing  number 
of  pieces  of  blue  glass  between  the  eye  and  the  sun,  or 
by  examining  different  thicknesses  of  litmus  solution  by 
transmitted  light. 

252.  Almost  All  Opaque  Substances  are  Transparent  in 
Very  Thin  Sections. — Since  no  substance  is  known  which 
reflects  all  of  the  light  which  falls  upon  it,  some  light 
must  enter  every  substance,  even  those  substances  called 
opaque.  A  substance  is  called  opaque  when  the  light 
which  enters  it  is  not  able  to  penetrate  far  into  the  sub- 
stance before  it  is  stopped  (absorbed).  Since  the  light 
which  enters  the  opaque  substance  must  penetrate  some 
distance  before  it  is  absorbed,  light  may  pass  through  any 


BADIANT  ENERGY,   HEAT  AND   LIGHT     273 

opaque  substance  if  a  thin  enough  layer  of  the  substance 
can  be  obtained.  Hence  every  substance  is  transparent 
to  some  degree. 

In  practice,  it  has  been  found  that  it  is  very  easy  to  se- 
cure sections  of  wood  which  are  thin  enough  to  be  trans- 
parent. There  is  such  little  difficulty  in  securing  trans- 
parent sections  of  even  the  blackest  rocks  that  one  of  the 
courses  in  geology  (petrography)  consists  in  a  study  of 
the  mineralogical  composition  of  rocks  by  means  of  a 
microscopical  investigation  of  thin  transparent  sections. 

Even  gold  is  transparent,  when  it  is  secured  in  thin 
films.  Gold  reflects  a  yellow  color,  but  if  thin  films  of 
gold  are  placed  between  pieces  of  glass  in  front  of  the 
light  of  a  magic  lantern,  they  will  transmit  green  light. 
Very  thin  films  of  silver  transmit  blue  light. 

Therefore,  when  we  say  that  a  substance  is  opaque  we 
mean  that  an  ordinary  thickness  of  the  substance  is  opaque. 
Sufficiently  thin  layers  of  this  substance  are  almost  cer- 
tain to  be  transparent. 

253.  The  Colors  of  Opaque  Objects  Partly  Due  to  In- 
ternal Reflection. — When  light  is  thrown  upon  glass,  a 
part  is  reflected  by  the  surface  of  the  glass,  and  a  part, 
after  it  has  passed  through  the  glass,  is  reflected  by  the 
air  at  the  rear  of  the  glass.  The  result  is  that,  if  a  lighted 
candle  be  placed  before  a  thick  piece  of  plate-glass,  two 
images  of  the  candle  are  formed.  The  front  image  is  due 
to  the  reflection  of  light  by  the  front  surface  of  the  glass ; 
the  rear  image  is  due  to  the  reflection  of  light  by  the  air 
in  contact  with  the  rear  of  the  glass. 

If  the  glass  is  colorless,  the  light  is  not  altered  in  color 
as  it  passes  through  the  glass  and  back  again,  after  reflec- 
tion, to  the  side  of  the  glass  from  which  it  came.  But  if 
the  glass  is  colored,  for  instance  if  the  glass  is  red,  the 


274  ELEMENTARY   PHYSICS 

light  which  traverses  the  thickness  of  the  glass  and  then 
returns  is  red,  for  red  glass  absorbs  all  the  other  colors. 

In  the  same  manner,  the  light  entering  many  so-called 
opaque  objects  is  reflected  by  different  materials  within  a 
very  short  distance  of  the  surface,  and  so  returns  to  the 
original  surface,  containing  only  those  rays  which  thin 
sections  of  the  opaque  objects  are  not  able  to  absorb. 

It  is  probable  that  the  colors  of  all  opaque  objects  are 
due  to  partial  absorption  of  the  colors  which  fall  upon  them. 
While  these  bodies  are  called  opaque,  the  light  in  reality 
penetrates  for  a  short  distance  and  is  reflected  again  to- 
ward the  surface,  but  only  the  light  not  absorbed  during 
the  transition  again  reaches  the  surface,  and  gives  the 
characteristic  color  of  the  object.  Any  color  reflected 
from  the  actual  surface  of  an  object  is  probably  reflected 
practically  unchanged. 

254.  Color  of  Pigments. — Copper  sulphate  and  potas- 
sium bichromate  both  form  transparent  crystals.  Copper 
sulphate  transmits  blue  chiefly  ;  it  absorbs  most  of  the 
red,  orange,  and  yellow,  but  transmits  a  little  green 
(Fig.  110,  A).  Potassium  bichromate  transmits  orange- 
red  chiefly,  the  color  noticed  on  examining  the  crystals ; 
it  absorbs  most  of  the  blue  and  violet,  but  transmits  a 
little  green  (Fig.  110,  B).  The  solutions  absorb  and  trans- 
mit the  same  colors  as  the  crystals.  The  result  is  that 
when  a  copper  sulphate  solution  is  held  in  front  of  a  po- 
tassium bichromate  solution  the  only  color  which  is  trans- 
mitted to  any  considerable  extent  by  both  solutions  is 
green,  and  hence  the  light  seen  through  both  solutions 
appears  green,  and  a  mixture  of  these  solutions  also  ap- 
pears green  (Fig.  110,  C,  D). 

If  a  very  small  quantity  of  finely  powdered  potassium 
bichromate  is  added  to  a  larger  quantity  of  finely  pow- 


RADIANT  ENERGY,   HEAT  AND   LIGHT     275 

dered  copper  sulphate,  and  the  mixture  is  then  thoroughly 
stirred,  a  slight  change  of  color  is  noticeable.  If  the  pro- 
portion of  potassium  bichromate  is  gradually  increased, 
the  mixture  finally  becomes  a  distinct  light  green,  or  ap- 
ple-green. Any  light  which  strikes  the  mixture  pene- 
trates it  for  some  depth  before  it  is  reflected  back  to  the 
surface,  and,  since  during  its  passage  through  the  mixt- 


r o       y        gr        bl          in 


Colors  transmuted 
by  copper  sulphate 


FIG.  HO. 

ure  all  the  colors  except  green  are  absorbed,  the  mixture 
appears  green.  Mix  in  the  same  manner  chrome  yellow 
and  ultramarine  blue  (§§  244  and  253). 

255.  The  Relative  Transmission  of  Visible  and  of  Ultra- 
Red  Waves  Varies  in  Different  Substances. — Since  most 
substances  are  not  able  to  transmit  equally  well  all  of 
the  prismatic  colors  which  form  the  visible  part  of  the 
spectrum,  it  is  probable  that  most  substances  cannot 


276  ELEMENTARY  PHYSICS 

transmit  all  of  the  waves  formed  by  the  invisible  parts  of 
the  spectrum.  This  fact  is  shown  to  a  certain  extent  by 
the  following  observations. 

Rock  salt  appears  to  transmit  the  invisible  ultra-red 
waves  about  as  well  as  the  visible  waves  of  the  spectrum. 
Hence  prisms  used  to  investigate  the  ultra-red  part  of 
the  spectrum  are  usually  made  of  rock  salt.  A  solution 
of  iodine  in  carbon  bisulphide  transmits  none  of  the  vis- 
ible waves,  but  transmits  a  very  considerable  quantity  of 
the  invisible,  ultra-red  waves.  Alum  and  ice  transmit 
almost  none  of  the  invisible  ultra-red  waves,  but  transmit 
most  of  the  visible  waves  of  the  spectrum.  Since  most 
of  the  heat  given  out  by  the  light  in  a  stereopticon 
comes  usually  from  the  invisible  ultra-red  waves,  a  solu- 
tion of  alum  in  water  is  often  used  to  shield  lenses  and 
slides  when  images  of  microscopic  objects  are  thrown 
upon  a  screen  by  means  of  a  magic  lantern  or  stereop- 
ticon. 

Calcite,  glass,  and  quartz  transmit  very  little  of  the 
heat  due  to  those  invisible,  ultra-red  waves  whose  rate  is 
low,  but  they  transmit  more  of  the  more  rapid  ultra-red 
waves  which  lie  nearer  the  visible  spectrum,  and  they 
transmit  most  of  the  visible  waves. 

256.  Good  Reflectors  of  Visible  and  Invisible  Waves  are 
Poor  Absorbers  of  Heat. — Those  visible  and  invisible  waves 
which  fall  upon  a  substance  and  which  are  neither  re- 
flected nor  transmitted,  seem  to  disappear  entirely.  They 
are  said  to  be  absorbed.  What  becomes  of  these  waves  ? 
They  increase  the  heat  of  the  substance  through  which 
they  pass  (§  257). 

Those  substances,  therefore,  which  reflect  the  least  light 
(and  which  probably  also  reflect  the  smallest  quantity 
of  the  invisible  waves)  ought  to  show  the  greatest  gain 


RADIANT  ENERGY,   HEAT  AND   LIGHT     277 

in  heat.  This  may  be  verified  by  an  interesting  experi- 
ment. Place  upon  the  snow,  in  direct  sunlight,  three 
pieces  of  cloth,  alike  in  every  particular  except  in  color. 
Suppose  the  colors  chosen  are  white,  red,  and  black.  The 
snow  will  melt  most  rapidly  around  the  black  cloth,  and 
least  readily  around  the  white  cloth.  The  black  color  is 
evidently  due  to  the  absorption  of  most  of  the  light  of 
the  sun,  and  white  is  due  to  the  least  absorption,  since  it 
evidently  reflects  all  the  colors  to  a  considerable  extent. 
Therefore,  the  black  cloth  becomes  warm  more  quickly 
than  the  white  cloth.  What  colors  should  be  worn  during 
summer  ?  During  winter  ? 

Opaque  bodies,  if  they  are  sufficiently  thick,  absorb  all 
the  light  which  enters  them,  with  the  exception,  of  course, 
of  that  part  of  the  light  which  suffers  interior  reflection 
very  near  the  surface  of  the  body  (§251).  Lamp-black 
seems  to  absorb  all  waves  (ultra-red  and  visible),  and  to 
reflect  none.  Most  metals  absorb  about  13  or  14  per 
cent,  of  both  the  ultra-red  and  of  the  visible  waves.  They 
must,  therefore,  be  excellent  reflectors  of  both  kinds  of 
waves.  For  that  reason  certain  metals  were  formerly 
used  extensively  for  mirrors.  So-called  glass  mirrors  are 
panes  of  glass  with  a  metallic  coating  (chiefly  mercury) 
upon  the  back.  This  coating  does  most  of  the  reflecting. 
The  image  reflected  by  the  front  of  the  glass  is  rarely 
noticed. 

257.  Good  Transmitters  of  Visible  and  Invisible  Waves 
are  Poor  Absorbers  of  Heat. — It  has  already  been  shown 
that  substances  vary  considerably  in  their  ability  to  trans- 
mit all  the  waves  which  form  the  visible  and  the  invisible 
parts  of  the  spectrum.  Those  substances  which  trans- 
mit most  of  the  waves,  especially  of  those  invisible  ultra- 
red  waves  which  give  evidence  of  the  greatest  intensity 


278  ELEMENTARY   PHYSICS 

of  heat,  will  necessarily  absorb  but  little  heat  and  will 
therefore  remain  comparatively  cold  even  in  direct  sun- 
light. Salt  is  one  of  the  best  transmitters  and  therefore 
is  a  poor  absorber  of  heat.  Glass  transmits  much  less, 
and  therefore  absorbs  much  more  of  the  heat.  Alum, 
however,  transmits  but  little  heat  and  absorbs  so  much 
that  it  is  used  to  protect  other  bodies  from  heat  (§  255). 
Most  of  the  rock-forming  minerals  absorb  more  heat 
than  they  transmit.  Air,  on  the  contrary,  absorbs  but 
little  heat.  This  fact  may  be  recognized  easily  on  a  hot 
summer's  day  by  placing  the  hand,  which  is  already  in 
contact  with  the  air,  on  dry  ground  which  has  been  ex- 
posed to  direct  sunlight.  The  contrast  between  the  tem- 
perature of  the  ground  and  the  temperature  of  the  air  is 
great. 

The  atoms  forming  the  surface  of  the  body  may  be  set 
in  vibration  by  the  waves  set  up  in  the  ether  so  readily 
that  the  face  exposed  to  direct  sunlight  may  feel  burning 
hot,  though,  at  the  same  time,  the  air  in  contact  with  the 
face  may  have  a  considerably  lower  temperature.  The 
instant  an  umbrella  is  used  as  a  screen,  the  air  is  felt  to 
be  cooler. 

Water  vapor  is  a  much  better  absorber  of  heat  than  air, 
although  both  are  very  transparent  to  light.  Therefore, 
moist  air  becomes  hot  much  more  quickly  than  dry  air. 

258.  Explanation,  by  Means  of  the  Ether,  of  the  Connection 
between  Good  Absorption  and  Poor  Transmission. — In  at- 
tempting to  explain  the  evident  connection  between  good 
absorption  and  poor  transmission  of  waves,  or  between 
poor  absorption  and  good  transmission,  the  following  as- 
sumptions are  made : 

When  atoms  do  not  obstruct,  to  any  considerable  ex- 
tent, the  transmission  of  vibrations  through  the  ether 


RADIANT  ENERGY,    HEAT  AND   LIGHT      279 

within  which  the  atoms  are  imbedded,  the  vibrations  are 
transmitted  through  the  ether  with  very  little  loss  of  en- 
ergy ;  but  the  atoms  are  not  set  into  very  active  vibration. 
Substances  of  this  kind  are  good  transmitters  but  poor 
absorbers  of  heat. 

However,  when  atoms  obstruct  strongly  the  transmis- 
sion of  these  waves,  the  energy  of  the  waves  in  the  ether 
is  considerably  diminished.  If  a  sufficient  number  of 
these  obstructing  atoms  be  present,  the  energy  of  the 
waves  passing  through  the  ether  within  the  interior  of 
the  body  formed  by  these  atoms  becomes  so  small  that 
the  vibrations  of  the  ether  practically  die  out  there.  But, 
during  this  time,  the  obstructing  atoms  begin  to  vibrate 
violently,  and  are  said  to  have  absorbed  the  heat.  Sub- 
stances of  this  kind  are  good  absorbers  but  poor  trans- 
mitters of  heat. 

259.  Good  Absorbers    are    Good    Radiators   of    Heat.— 
Investigators  have  shown  by  experiment  that  any  body 
which  absorbs  heat  readily  gives  it  out   again  readily. 
Such  a  body  is  said  to  radiate  the  heat  which  it  has  ab- 
sorbed.   The  sand  of  the  desert  of  Sahara,  which  becomes 
heated  quickly  in  daytime,  cools  down  rapidly  at  night. 
The  dark  soil  in  our  climate  absorbs  heat  readity.     When 
the  sky  is  cloudless  at  night,  the  soil  again  gives  up  the 
heat  readily  ;  but  if  the  sky  be  covered  by  clouds,  this 
heat  will  be  absorbed  by  the  clouds,  and  then  given  back 
again  to  the  air  beneath,  so  that  cloudy  nights  are  not  as 
cold  as  cloudless  nights. 

260.  The   Chemical  Effect  of  Waves  Belonging  to   Dif- 
ferent Parts  of  the  Spectrum. — If  silver  nitrate  is  placed 
upon  some  animal  or  vegetable  substance  and  exposed 
to  sunlight,  the  effect  of  the  sunlight  is  to  produce  cer- 
tain chemical  reactions  between  the  silver  nitrate  and 


280  ELEMENTAEY   PHYSICS 

the  other  substances,  and  these  reactions  give  rise  to  a 
change  in  color.  In  a  similar  manner,  the  light  of  the  sun, 
falling  upon  the  various  silver  compounds  used  in  photog- 
raphy, causes  a  chemical  reaction  which,  when  it  is  fol- 
lowed by  other  chemical  changes  during  the  development 
pf  the  plate,  produces  a  black  color. 

It  might  be  supposed  from  this  that  all  objects  which 
reflect  much  light  will  appear  black  after  the  plate  ex- 
posed in  the  camera  is  developed.  However,  when  the 
effects  of  various  colors  upon  the  photographic  plate  are 
studied,  it  is  found  that  some  colors  produce  much  less 
effect  upon  silver  compounds  than  other  colors  do.  The 
chemical  action  is  usually  greatest  when  the  plate  is  ex- 
posed to  violet  or  bluish-violet  colors,  and  it  diminishes 
as  colors  nearer  the  red  end  of  the  spectrum  are  used. 
When  the  effects  of  the  invisible  waves  beyond  the  violet 
end  of  the  spectrum  are  investigated,  it  is  discovered  that 
these  effects  diminish  in  intensity,  as  the  distance  of 
these  invisible  waves  from  the  violet  end  of  the  spectrum 
increases. 

When  the  effect  of  light  upon  other  substances  which 
are  not  silver  compounds  is  examined,  very  different  re- 
sults are  obtained.  Certain  compounds  of  iron  are  most 
sensitive  to  waves  which  lie  beyond  the  extreme  red  end 
of  the  spectrum.  Each  chemical  compound  which  is 
likely  to  undergo  chemical  reactions  when  it  is  exposed 
to  light,  evidently  absorbs  waves  of  a  certain  rate  best, 
so  that  the  chemical  action  depends  not  only  upon  the 
frequency  of  the  waves,  but  also  upon  the  particular  sub- 
stance exposed  to  these  waves. 

261.  Effect  of  Distance  of  an  Object  upon  the  Distance 
of  the  Image  from  the  Lens.— Hold  a  double  convex  lens, 
or  a  lens  both  of  whose  faces  are  convex,  a  considerable 


RADIANT   ENERGY,   HEAT  AND   LIGHT     281 


distance  back  from  the  window.  Behind  the  lens  place  a 
piece  of  paper  in  such  a  position  that  the  light  reflected 
from  some  building  may  pass  through  the  lens  and  strike 
the  paper.  Move  the  paper  back  and  forth,  until  some 
position  is  found  in  which  a  fairly  distinct  image  of  the 
building  is  seen  upon  the  paper. 

A  much  better  image  may  be  secured  in  the  following 
manner :  Cut  a  hole  in  one  end  of  a  box,  of  such  a  shape 
and  size  that  a  lens  may  be  inserted  without  permitting 
any  light  to  pass  between  the  margin  of  the  lens  and  the 


TIG,  Hi. 

wood  surrounding  the  hole  (Fig.  111).  Instead  of  paper 
use  a  piece  of  ground  glass,  fastened  in  a  vertical  frame, 
which  can  be  moved  backward  and  forward  within  the  box. 
The  end  of  the  box  opposite  the  lens  should  be  left  open, 
so  that  the  image  upon  the  ground  glass  may  be  readily 
seen.  Secure  an  image  of  the  same  building  and  record 
the  distance  of  the  ground  glass  from  the  lens.  Now 
carry  the  box  to  a  point  much  nearer  to  the  building,  and 
move  the  screen  again  until  another  image  is  formed. 
The  screen  will  be  found  to  occupy  a  position  farther 
from  the  lens.  Keplace  the  screen  in  the  position  it  oc- 


282  ELEMENTARY   PHYSICS 

cupied  when  the  first  image  was  made.  Now  carry  the 
box  to  a  position  much  farther  removed  from  the  building 
than  any  position  so  far  occupied.  In  order  to  secure  an 
image,  the  screen  must  be  moved  nearer  to  the  lens.  The 
positions  occupied  by  the  screen  at  times  when  distinct 
images  are  formed  are  called  foci  (singular,  focus).  From 
the  observations  here  recorded,  it  may  be  seen  that  there 
are  a  great  many  positions  or  foci  at  which  images  may  be 
formed  by  the  same  lens,  and  that  the  distance  of  each 
focus  from  the  lens  depends  upon  the  distance  of  the  ob- 
ject from  the  lens.  In  photographic  cameras  the  ground 
glass  often  occupies  a  fixed  position  and  the  lens  is  moved 
back  and  forth  until  the  image  falls  upon  the  screen. 

Instead  of  the  building  a  gas  flame  or  candle  may  be 
used  as  an  object,  and  a  piece  of  white  paper  will  serve  as 
a  screen.  The  apparatus  being  placed  upon  a  table,  dif- 
ferent positions  may  be  given  to  the  lens,  and  the  screen 
may  be  moved  back  and  forth  until  a  distinct  image  is 
produced  upon  the  paper. 

262.  Effect  of  Distance  of  an  Object  from  the  Lens  upon 
the  Size  of  the  Image  on  the  Screen. — If  the  size  of  the 
image  be  noted  in  the  three  cases  described  in  the  preced- 
ing paragraph,  it  will  be  found  that  the  image  is  smaller 
when  the  object  is  farther  from  the  lens,  and  larger  when 
the  object  is  nearer  to  the  lens.  In  order  to  secure  an  im- 
age of  an  object  very  near  to  the  lens,  it  may  be  necessary 
to  move  the  screen  back  quite  a  distance  from  the  lens. 
For  this  reason  it  is  impossible  to  secure  a  good  photo- 
graph if  the  camera  be  held  very  near  to  an  object,  unless 
the  screen  or  photographic  plate  can  be  moved  quite  a  dis- 
tance from  the  lens.  These  facts  can  be  most  readily  illus- 
trated by  using  the  gas  flame  or  candle  as  an  object  and 
the  paper  as  a  screen  as  suggested  in  the  last  paragraph. 


RADIANT   ENERGY,   HEAT   AND   LIGHT     283 

263.  Effect  of  Convexity  of  Lens  upon  the  Distance  of 
the  Image  from  the  Lens.— If  the  lens  in  the  opening  of 
the  box  is  replaced  by  one  having-  a  greater  convexity,  and 
the  box  is  carried  to  each  of  the  three  positions  occupied 
before,  another  series  of  images  is  secured.     But,  in  each 
case,  the  image  is  found  nearer  to  the  lens  than  it  was 
when  the  less  convex  lens  was  used.     If  the  experiments 
be  repeated  with  a  lens  of  less  convexity,  the  distance  of 
the  image  from  the  lens  will  be  found  to  be  greater. 

If  a  sufficient  number  of  lenses  of  different  convexity 
are  at  hand,  it  is  possible  to  secure  an  image  of  any  ob- 
ject, no  matter  at  what  distance,  without  moving  the 
screen.  If  the  object  is  very  far  away,  it  is  necessary  to 
use  a  lens  of  slight  convexity.  If  the  object  is  near  at 
hand,  a  lens  of  greater  convexity  must  be  used. 

264.  Reason  why  the  Image  on  the  Screen  is  Inverted. — 
It  is  evident  that  in  the  preceding  experiments  any  light 
coming  from  the  top  of  the  building  and  passing  through 
the  lens  strikes  the  screen  near  its  lower  margin.     Any 
light  from  the  bottom  of  the  building  strikes  near  the  top 
of  the  screen.     Light  from  the  left  side  of  the  building 
reaches  the  right  side  of  the  screen.     In  other  words,  the 
image  produced  upon  the  screen  is  inverted. 

265.  The  Image  Most  Distinct  when  the  Margin  of  the 
Lens  is  Not  Used.— When  the  entire  surface  of  the  lens  is 
used,  it  is  found  that  the  image  is  brighter,  but  not  as 
distinct.     If  the  margin  of  the  lens  is  covered  in  some 
way,  a  considerable  quantity  of  light  is  excluded.     The 
image  is,  therefore,  not  so  bright,  but  it  is  much  more 
sharp  or  distinct.     For  this  reason,  cameras  are  provided 
with  stops  or  diaphragms,  which  make  it  possible  to  use 
only  the  central  part  of  the  lens  in  bright  weather.     If 
objects  are  in  motion,  so  that  it  is  necessary  to  take  an  in- 


284  ELEMENTARY  PHYSICS 

stantaneous  picture,  it  may  be  necessary  to  use  the  entire 
lens  to  secure  a  good  photograph  in  a  very  short  time. 
Moreover,  if  it  be  very  dark,  the  entire  lens  must  be  used 
in  order  to  secure  sufficient  light,  if  the  exposure  is  to  oc- 
cupy only  a  short  time. 

266.  Structure  of  the  Eye. — The  essential  part  of  the 
eye  consists  of  a  lens,  L,  enclosed  within  the  eyeball,  near 
the  front,  and  a  screen,  called  the  retina,  R  R,  which  lines 
the  inner  surface  of  the  eyeball  (Fig.  112).  The  image 
formed  by  the  lens  must  fall  upon  the  retina.  If  the 

image  produced  up- 
on the  retina  is  dis- 
tinct, the  object  is 
seen  distinctly.  The 
position  of  the  screen 
is  fixed.  The  lens  can 
be  moved  only  a  lit- 

Optic  Nerve  .,       ,,  ,  ,        , 

tie  forward  or  back- 
ward. There  is  only 
one  lens,  and  it  must 
serve  to  produce  an 

FlG  ^  image  of  any  object, 

no    matter    at    what 

distance  from  the  eye.  This  can  be  accomplished  only 
when  the  convexity  of  the  lens  can  be  changed  at  will,  so 
that  at  times  it  can  serve  as  a  moderately  convex  lens  and 
at  other  times  as  a  strongly  convex  one.  This  changing 
is  made  possible  by  a  series  of  muscles.  It  is  possible 
to  perceive  the  effort  of  the  eye  to  change  the  convexity 
of  the  lens,  when  objects  at  very  different  distances  are 
examined  in  succession.  Near  objects  require  a  greater 
convexity  of  the  lens  and  distant  objects  a  smaller  con- 
vexity. If  the  proper  muscles  are  not  able  to  give  the 


RADIANT  ENERGY,   HEAT  AND   LIGHT     285 


proper  convexity  to  the  lens  in  the  eye,  the  image  will 
not  appear  where  it  is  needed.  The  lens  will  form  an 
image  in  front  or  behind  the  retina.  In  this  case  the 
image  will  be  blurred  or  indistinct.  If  the  lens  cannot 
be  made  sufficiently  convex,  convex  spectacles  must  be 
worn.  If  the  lens  be  too  convex,  concave  spectacles  must 
be  worn  to  counteract  this  effect.  Some  persons  need 
spectacles  only  when  they  are  reading.  This  means  that 
the  eye  can  accommodate  itself  to  all  conditions,  except- 
ing when  a  very  convex  lens  is  necessary. 

Immediately  in  front  of  the  lens  is  suspended  a  circu- 
lar curtain  or  diaphragm,  called  the  iris,  1  1,  which  leaves 
a  circular  opening,  thepupil,Pf  in  front  of  the  lens.  This 
opening  may  be  enlarged  or  diminished,  so  as  to  admit 
the  use  of  more  or  less  of  the  lens,  according  to  the 
amount  of  light  furnished  by  the  object  to  be  examined. 
In  a  dark  room  the  opening  through  the  iris  (the  pupil) 
is  usually  large.  In  very  bright  light,  the  opening  is 
much  reduced  in  size. 

267.  The  Sensation  of  Sight.—  The  retina,  upon  which 
the  image  falls,  is  a  transparent  layer  lining  the  interior 

of  the  eyeball  back  of 

^lj        I 


the  crystalline  lens. 
The  space  between 
the  retina  and  the 
lens  and  the  space 
between  the  lens  and 
the  front  of  the  eye- 
ball is  occupied  by 
transparent  liquids. 
only 


FIG.  113. 


Although  the  retina  forms  a  layer 
of  an  inch  thick,  it  has  a  rather  complicated 
structure,  so  that,  for  convenience  of  description,  it  is 
usually  divided  into  ten  layers  (Fig.  113).  These  will  be 


286  ELEMENTARY   PHYSICS 

found  well  described  in  the  larger  books  on  physiology. 
For  our  purpose,  it  is  sufficient  to  know  that  the  nerves 
of  sight  enter  as  a  bundle  (optic  nerve)  at  the  rear  of  the 
eye  (Fig.  112),  directly  opposite  the  lens,  and,  after  pass- 
ing through  all  the  coats  of  the  eye,  spread  out,  so  as  to 
reach  every  portion  of  the  retina. 

That  part  of  the  thickness  of  the  retina  which  is  far- 
thest removed  from  the  interior  of  the  eye  contains  a  layer 
of  bodies  described  as  rods  and  cones  (Fig.  113).  The 
vibrations  set  up  in  these  rods  and  cones  are  believed  to 
be  the  cause  of  the  sensation  of  sight.  A  number  of  facts 
indicates  this,  the  most  interesting  among  which  is  the 
following  :  The  smallest  star  which  can  be  seen  in  the  sky 
is  one  whose  apparent  diameter  is  so  small  that  lines 
drawn  from  the  extremities  ,of  this  diameter  to  the  ob- 
server on  earth  would  form  an  angle  of  only  60  seconds. 
The  smallest  distance  which  must  exist  between  lines  so 
that  they  may  still  be  recognized  as  separate  lines  is  such 
that  lines  connecting  these  lines  with  the  eye  give  an 
angle  which  varies  from  64  to  73  seconds.  The  sizes  of 
the  images  formed  upon  the  retina  of  the  eye  in  these 
cases  have  been  calculated,  and  it  has  been  determined 
that  the  image  of  the  star  would  have  a  diameter  of  .00017 
of  an  inch,  while  the  distance  between  the  lines,  in  the 
image,  would  vary  between  .00018  and  .00021  of  an  inch. 
Now  the  diameter  of  the  rods  and  the  cones  varies  be- 
tween .00018  and  .00022  of  an  inch.  The  exact  character 
of  the  connection  between  the  rods  and  cones  and  the 
nerves  of  sight  has  not  been  fully  established. 

268.  Sensitiveness  of  the  Eye  to  Different  Colors  of  the 
Spectrum. — If  the  intensity  of  the  different  colors  seen  in 
the  spectrum  produced  by  sunlight  be  examined,  it  will 
be  found  that  the  greatest  intensity  of  color  is  shown  by 


RADIANT  ENERGY,   HEAT  AND   LIGHT     287 

the  tints  nearest  the  yellow  part  of  the  spectrum.  This 
is  due  to  the  fact  that  the  nerves  of  our  eyes  are  more 
sensitive  to  these  tints  than  to  any  others.  They  are  less 
and  less  sensitive  to  the  other  tints  as  these  recede  either 
toward  the  red  or  toward  the  violet  end  of  the  spec- 
trum. Some  experiments  seem  to  show  that  the  eyes  are 
more  sensitive  to  the  blue  colors  than  to  the  red  colors  at 
the  extreme  end  of  the  red  part  of  the  spectrum. 


CHAPTER  VII 
MAGNETISM  AND  ELEOTEICITY 

269.  Natural  Magnets. — It  was  observed  by  the  ancients 
from  the  remotest  antiquity  that  certain  hard  and  very 
heavy  black  stones  possess  the  remarkable  property  of 
attracting  small  pieces  of  iron.     These  stones  were  be- 
lieved to  possess  a  magical  power,  and  are  mentioned  by 
many  ancient  writers.     Although  they  excited  much  won- 
der, no  practical  use  was  made  of  them  until  about  l^ie 
twelfth  century,  when  it  was  discovered  that,"  if  one  of 
these  stones  is  suspended  so  as  to  be  free  to  turn  in  any 
horizontal  direction,  it  always  finally  comes  to  rest  in  the 
same  position — one  end  pointing-  North,  the  other  South. 

It  is  now  known  that  these  stones  are  pieces  of  mag- 
netic iron  ore,  and,  where  this  ore  occurs  in  abundance,  it 
is  mined  in  order  to  secure  the  iron  which  is  its  chief  con 
stituent.  Its  chemical  composition  is  Fe3O4.  On  ac- 
count of  its  magnetic  properties  the  ore  is  also  called 
magnetite. 

270.  Artificial  Magnets. — It  is  possible  to  give  to  a  piece 
of  steel  the  same  properties  as  those  possessed  by  mag- 
netite.   If  the  same  end  of  a  piece  of  magnetite  is  rubbed 
always  in  the  same  direction  upon  a  knitting  needle,  the 
needle  will  attract  iron  filings  and  small  tacks,  and,  if  sus- 
pended by  a  thread,  it  will  point  approximately  North 
and  South  (Fig.  114).    By  means  of  powerful  electric  cur- 
rents it  is  possible  to  give  these  properties  even  to  large 

288 


MAGNETISM   AND   ELECTRICITY 


289 


S. 


FIG.  114. 


bars  of  steel.  With  these  steel  bars  much  heavier  weights 
of  iron  or  steel  can  be  raised  than  can  be  lifted  by  any 
piece  of  magnetic  iron  ore  of  the 
same  weight.  The  property  of 
attracting  iron  or  steel  and  of  as- 
suming a  North  and  South  direc- 
tion is  known  as  magnetism.  Bars 
of  steel  possessing  these  proper- 
ties are  known  as  magnets.  Pieces 
of  magnetite  are  sometimes  re- 
ferred to  as  natural  magnets. 

271.  Soft  Iron  Readily  Loses  its  Magnetism. — It  is  pos- 
sible to  give  magnetic  properties  even  to  an  ordinary 
piece  of  iron,  but  ordinary  iron  does  not  retain  magnetism 
wejl.  Magnetize  a  piece  of  iron  wire  about  a  foot  long  and 
one-eighth  of  an  inch  in  diameter  by  rubbing  it  with  a 
strong  steel  magnet.  Dip  the  end  in  iron  filings  and  no- 
tice the  size  of  the  cluster  which  adheres  (Fig.  115,  A). 
Then  bend  or  twist  the  wire  roughly  and  dip  the  same 
end  into  the  filings.  The  magnetism  is  practically  gone 
(Fig.  115,  B).  Some  kinds  of  iron  can  be  bent  or  cut  so 
much  more  readily  than  others  that  they  are  called  soft. 
The  softest  kinds  of  iron  lose  their  magnetism  at  the  slight- 

^=^~~==-        ^  6st  jar  as  soon  as 

A  j]          the    magnetizing 

J|  J|         agency    has    been 

removed.  Steel  is 
the  only  material 
which  retains  mag- 
netism well ,  and  is, 
therefore,  the  only 
substance  used  for  permanent  magnets,  although  even 
steel  loses  its  magnetism  if  roughly  handled  or  heated. 


FIG.  115. 


290  ELEMENTARY  PHYSICS 

272.  Only  Iron  and  Steel  Can  be  Strongly  Magnetized. — 
Iron  and  steel  are  the  only  substances  which  can  be  mag- 
netized strongly,  and  they  are  also  the  only  substances 
which  are  strongly  attracted  by  magnets.     In  the  study 
of  the  ordinary  properties  of  magnetism,  all  other  sub- 
stances may  be  ignored. 

273.  Magnetic  Transparency. — All  substances  which  are 
not  readily  attracted  by  magnets  permit  magnetism  to 
act  readily  through  them.     They  are  therefore  said  to  be 
magnetically  transparent.     It  is  impossible  to  pick  up 
even  a  very  small  particle  of  iron  by  means  of  a  piece  of 
glass,  but,  if  a  small  pane  of  glass  is  held  against  the 
lower  end  of  a  magnet,  the  force   of  the  magnet  acts 
through  the  glass,  and  it  is  possible  to  pick  up  tacks 
placed  immediately  beneath.      The  tacks  are  drawn  up 
against  the  glass,  but  the  instant  the  magnet  is  removed 
tke  tacks  drop.     Almost  all  other  substances  are  magnet- 
ically transparent.     Iron  and  steel  are  the  only  conspicu- 
ous exceptions.      Substances  are  less  transparent  mag- 
netically in  proportion  as  they  give  evidence  of  greater 
magnetic  properties. 

274.  Magnetic  Poles. — When  magnets  are  dipped  in  fil- 

ings or  tacks  it  is  found 
that  the  filings  or  tacks 
adhere  in  tufts,  and  that 
the  tufts  are  longest  and 
most  abundant  along 
the  edges  near  the  ends 
(Fig.  116).  Magnetism  is 
FlG  116  therefore  strongest  near 

the  ends  of  magnets  and 

these  ends  are  called  poles.     Every  magnet  has  two  poles. 

Both  poles  can  pick  up  the  same  quantity  of  tacks,  never- 


MAGNETISM  AND  ELECTRICITY  291 

theless  they  must  be  quite  different  from  one  another, 
since  one  end  always  turns  North  and  the  other  South. 

That  there  is  a  difference  between  the  poles  of  a  mag-net 
can  be  shown  in  another  manner.  Mark  with  chalk  or 
paint  those  ends  of  two  magnets  which 
point  North.  Usually  these  ends  are 
already  marked  with  the  letter  N. 
Suspend  one  magnet  horizontally  by 
means  of  a  thread  and  hold  near  its 
north-seeking-  end  the  north-seeking1 
end  of  the  other  magnet.  The  two 
poles  actually  repel  one  another  (Fig. 
117).  This  result  would  certainly  not 
be  expected  from  anything  previously 
stated.  Now  hold  the  south-seeking 
pole  near  the  north-seeking  pole  of  the 
magnet.  The  two  poles  seem  to  attract  each  other.  Re- 
peat the  experiment  in  as  many  forms  as  you  may  be  able 
to  devise  using  other  objects,  such  as  the  magnetic  needle 
in  a  compass,  and  the  same  result  will  always  be  found. 
We  may  state  these  facts  in  the  form  of  a  rule  as  fol- 
lows : — Like  poles  repel  each  other  ;  unlike  poles  attract  each 
other. 

275.  The  Earth  a  Magnet. — This  rule  opens  up  another 
question.  Why  do  all  magnets,  when  suspended,  point 
North  and  South  ?  This  is  a  very  practical  question, 
since  the  needle  in  every  compass  is  merely  a  small  mag- 
net. A  suspended  magnet  acts  as  though  some  south- 
seeking  pole  at  some  northern  part  of  the  earth  were 
drawing  the  north-seeking  pole  of  the  magnet  northward. 
Or,  as  if  some  north-seeking  pole  situated  at  some  south- 
ern point  on  the  earth's  surface,  were  attracting  the  south- 
seeking  pole  of  the  magnet  southward.  Now,  when  we 


292 


ELEMENTAEY  PHYSICS 


examine  the  directions  of  the  compass  needles  at  all  points 
of  the  earth's  surface,  we  find  that  in  the  northern  hemi- 
sphere their  north-seeking-  ends  all  point  in  a  general  way 
to  a  point  on  the  coast  of  North  America,  northwest  of 
Hudson  Bay  (Fig.  118).  And  in  a  general  way  in  the 
southern  hemisphere  the  needles  all  point  away  from  a 
locality  in  the  Antarctic  Ocean.  This  locality  does  not 
appear  to  be  directly  opposite  the  north  magnetic  pole 
(Fig.  118,  A).  Therefore,  there  is  apparently  a  south-seek- 
ing pole  near  the  North  end  of  the  earth  which  attracts 


,v^ca 


A  Pole 
tic  Pole 


The  Earth  a  Magnet. 
FIG.  118. 

the  north-seeking  pole  of  the  compass  needle  and  a 
north-seeking  pole  at  the  opposite  end  of  the  earth 
which  attracts  the  south-seeking  end  of  the  needle.  Ap- 
parently, the  earth  itself  is  a  magnet  with  its  poles  at 
the  North  and  South  ends.  Since  only  unlike  poles 
attract  each  other,  the  magnetism  of  the  poles  of  the 
earth  and  that  of  the  attracted  poles  of  the  compass 
needle  must  be  exactly  opposite.  This  may  be  shown 
experimentally  by  placing  a  compass  upon  its  side  and 
moving  it  back  and  forth  over  a  horizontal  magnet. 


MAGNETISM   AND   ELECTRICITY  293 

276.  The  Compass — The  essential  part  of  a  compass  is 
the  needle.  This  consists  of  a  small  magnetized  bar  of 
steel  supported  on  the  sharp  point  of  an  upright  pin  by 
means  of  a  cap  of  brass,  glass,  or  agate  attached  to  its 
centre.  In  this  manner  friction  is  reduced  to  a  minimum 
and  the  needle  can  be  moved  by  the  slightest  attraction. 
In  the  mariner's  compass  the  apparatus  is  so  arranged 
that  the  needle  will  remain  horizontal  notwithstanding 
the  roll  of  the  ship. 

It  was  discovered  by  Columbus  on  his  voyage  to  Amer- 
ica, that  the  compass  needle  did  not  point  to  exactly  the 
same  part  of  the  sky  while  sailing  across  the  Western 
Atlantic  as  it  did  in  Spain.  At  that  time  this  could  not 
be  explained,  but  now  we  know  that  the  magnetic  pole  in 
the  Northern  Hemisphere  is  not  situated  at  the  north 
pole  as  marked  in  our  geographies,  but  northwest  of  Hud- 
son Bay  on  the  peninsula  called  Boothia  Felix.  The  first 
pole  is  sometimes  called  the  geographical  pole,  and  the 
second  the  magnetic  pole. 

By  looking  at  a  geography  you  see  that  the  needle  must 
point  west  in  Greenland,  west  of  north  in  New  England, 
and  east  of  north  in  San  Francisco.  It  so  happens  that  in 
Ohio  the  needle  points  almost  exactly  north.  The  angle 
between  the  direction  of  the  compass  needle  and  the  true 
North  and  South  line  is  called  the  declination  of  the  nee- 
dle. The  declination  varies  at  different  parts  of  the  earth's 
surface,  and  in  order  that  a  navigator  may  know  in  what 
direction  true  North  is  located,  he  must  have  magnetic 
charts  which  give  the  declination  of  the  compass  needle 
at  all  points  of  the  sea.  Moreover,  it  is  necessary  to  have 
the  most  recent  charts,  since  the  magnetic  poles  of  the 
earth  shift  a  short  distance  toward  the  East  and  West  in 
the  course  of  a  number  of  years. 


294  ELEMENTARY  PHYSICS 

277.  Variation  in  the  Direction  of  a  Compass  Needle 
within  the  Field  of  a  Magnet. — If  a  compass  be  placed  at 
different  points  in  the  vicinity  of  a  bar  magnet,  the  direc- 
tion of  the  needle  will  vary  with  the  position  of  the  com- 
pass. The  space  within  which  the  bar  magnet  can  influ- 
ence the  direction  of  the  needle  of  a  compass  may  be  called 
the  field  of  the  magnet.  In  order  to  investigate  the  differ- 
ent directions  assumed  by  the  needle  within  this  field, 
place  the  bar  magnet  on  the  centre  of  a  large  piece  of  pa- 
per. Hold  the  compass  at  different  points  in  the  field, 
and  indicate  in  each  case  the  direction  of  the  needle  by 
means  of  an  arrow  drawn  directly  beneath  it.  The  point 
of  this  arrow  should  indicate  the  direction  assumed  by 
that  end  of  the  needle  which  points  North  when  not  in  the 
vicinity  of  the  bar  magnet. 

Continue  the  investigation  until  the  entire  field  for  a 
distance  of  at  least  six  inches  from  the  magnet  is  covered 
with  arrows.  It  then  becomes  apparent  that,  although 
the  direction  of  the  needle  varies  considerably  within  the 
field  of  the  magnet,  this  variation  is  in  accordance  with  a 
definite  plan.  The  arrows  around  one  end  of  the  magnet 
point  away  from  the  magnet,  while  those  around  the 
other  end  point  toward  it.  The  arrows  at  intermediate 
positions  assume  such  directions  that  it  is  possible  to 
connect  them  by  means  of  curved  lines  starting  out  from 
one  end  of  the  magnet  and  finally  entering  the  other. 
This  appearance  is  strengthened  by  continuing  the  pro- 
duction of  arrows  until  the  paper  is  densely  covered 
with  them.  If  long  lines  are  used  to  indicate  the  direc- 
tions of  the  needle,  the  arrows  frequently  cross  one 
another,  but  in  proportion  as  the  arrows  are  made 
shorter,  the  appearance  of  curved  lines  leaving  the  one 
pole  of  the  magnet  and  entering  the  other  is  increased. 


MAGNETISM  AND   ELECTKICITY  295 

If  a  pane  of  glass  is  placed  on  a  bar  magnet,  and  iron- 
filings  are  sifted  evenly  over  the  surface,  the  soft  iron-fil- 
ings will  temporarily  become  magnets.  A  gentle  tapping 
of  the  glass  will 

permit  the  filings    F'   '  ,  X 

to  arrange  them-     ;:       V 
selves    in    direc-  IJH&.I,. 

tions  correspond- 
ing  to  those 
which  would  be 
assumed  under 
the  same  condi-  FIG  119 

tions    by   minute 

magnets  (Fig.  119).  In  this  case  it  becomes  still  more 
apparent  that  the  directions  assumed  by  numerous  mi- 
nute magnetic  objects  under  the  influence  of  a  large  mag- 
net coincide  with  curves  which  may  be  drawn  leaving 
one  end  of  the  magnet  and  entering  at  the  other.  These 
curves  are  called  lines  of  force. 

278.  Magnetic  Lines  of  Force. — It  is  not  known  precisely 
why  magnetic  needles  arrange  themselves  in  accordance 
with  a  definite  plan  within  the  field  of  a  magnet.  Within 
recent  years,  however,  a  number  of  ideas  have  been  steadily 
gaming  ground,  and,  at  present,  these  ideas  largely  influ- 
ence current  views  on  this  subject. 

It  may  be  noticed,  in  the  first  place,  that  the  bar  mag- 
net influences  the  direction  assumed  by  the  compass  nee- 
dle placed  within  its  field,  although  not  in  direct  contact 
with  the  needle.  If  the  original  magnetic  force  resides 
in  the  bar  magnet,  this  force  must  in  some  manner  trav- 
erse the  air  in  order  to  reach  the  needle,  and  the  force 
must  be  conveyed  by  something,  by  some  medium.  Is  this 
medium  the  air  ?  This  may  be  easily  determined.  If  the 


296  ELEMENTAKY  PHYSICS 

bar  magnet  and  compass  needle  be  placed  under  a  bell-jar 
and  the  air  exhausted,  the  action  of  the  bar  magnet  upon 
the  needle  will  remain  exactly  the  same,  proving  that  air 
is  not  the  medium. 

Since  it  is  impossible  to  conceive  of  the  action  of  one 
object  upon  another  without  either  direct  contact  or  an 
intermediate  medium  conveying  the  force  indirectly,  it  is 
necessary  to  search  for  some  medium.  This  medium 
which  is  present  even  in  a  space  devoid  of  air  is  the 
ether,  whose  presence  has  already  been  found  necessary 
to  explain  the  various  phenomena  of  heat  and  light.  The 
theory  is  somewhat  as  follows  : 

In  some  manner  a  magnet  is  able  to  influence  the  ether 
in  the  air  surrounding  it.  The  space  within  which  the 
ether  is  affected  is  called  HLQ  field  of  the  magnet.  Within 
this  field  the  ether  is  strained,  and  the  direction  of  the 
strain  is  along  curved  lines.  Small  magnets  are  consid- 
erably affected  by  the  strain  produced  in  the  ether  by 
large  magnets,  and,  if  suspended  or  supported  so  as  to 
swing  freely,  are  forced  to  assume  positions  parallel  to 
the  direction  of  the  strain.  Another  way  of  expressing 
the  same  idea  is  to  say  that  the  needles  take  positions  at 
a  tangent  with  the  direction  of  the  strain  in  the  ether. 

A  compass  needle  may  therefore  be  used  to  determine 
the  direction  of  the  strain  of  the  ether  at  various  points 
within  the  field  of  the  magnet.  As  has  already  been 
shown  by  the  preceding  experiment,  the  direction  of 
this  strain  is  different  in  different  parts  of  the  field.  The 
continual  change  of  direction  in  the  strain  of  the  ether 
may  be  indicated  by  means  of  curved  lines.  These  lines 
are  drawn  so  as  to  leave  one  pole  of  the  magnet,  curve 
around  through  the  field  of  the  magnet,  and  enter  at  the 
other  pole.  Lines  drawn  in  this  manner  may  be  called 


MAGNETISM  AND   ELECTRICITY  297 

lines  of  force.  In  reality  they  serve  only  to  indicate  the 
direction  in  which  the  ether  is  believed  to  be  strained. 
There  are  no  actual  lines  present  in  the  ether.  But  draw- 
ings representing-  lines  of  force  traversing  the  field  around 
magnets  enable  the  student  quickly  to  grasp  the  chief 
properties  of  a  magnetic  field — the  direction  of  the  strain 
within  the  ether  and  the  resultant  direction  assumed  by 
any  small  magnet  placed  within  this  field. 

279.  Direction  Assumed  by  North-seeking1  Pole  of  Mag- 
netic Needle. — The  strain  in  the  ether  present  in  the  air 
surrounding  the  bar  magnet  determines  not  only  the  gen- 
eral direction  assumed  by  the  compass  needle,  but  deter- 
mines also  the  direction  in  which  the  north-seeking  pole  of 
the  needle  faces.  In  reality,  the  bar  magnet  does  not  affect 
the  north-seeking  pole  of  the  needle  more  than  the  south- 
seeking  pole,  but  it  is  convenient  to  determine  the  direc- 
tion in  which  a  needle  faces  by  stating  the  direction  in 
which  its  north-seeking  pole  faces.  If  the  lines  of  force 
mapping  the  different  directions  of  strain  in  the  ether 
be  followed  from  the  north-seeking  pole  of  the  bar  mag- 
net until  they  enter  the  south-seeking  pole,  the  north- 
seeking  pole  of  the  needle  at  every  part  of  this  entire 
path  will  point  forward,  while  the  south-seeking  end  of 
the  needle  points  backward. 

In  order  to  facilitate  the  ready  determination  of  the  di- 
rection assumed  by  the  compass  needle,  it  may  be  imag- 
ined that  the  lines  of  force  leave  the  north-seeking  end  of 
the  bar  magnet  and,  after  curving  through  the  field,  enter 
at  the  south-seeking  end.  In  this  case  the  rule  may  be 
given  that  the  north-seeking  ends  of  small  movable  mag- 
nets placed  within  the  field  of  a  larger  magnet  are  always- 
urged  to  face  in  the  direction  in  which  the  nearest  lines 
of  force  are  passing.  But  it  should  be  remembered 


298  ELEMENTARY   PHYSICS 

that  actual  lines  of  force  do  not  exist.  What  we  call  lines 
of  force  are  merely  useful  drawing's  by  means  of  which  we 
record  the  changes  in  direction  of  the  strain  in  the  ether. 
It  will  sometimes  be  found  convenient  to  refer  to  lines  of 
force  as  though  they  really  existed,  but  a  failure  to  under- 
stand their  real  nature  as  imaginary  lines  mapping  the 
direction  of  strain  in  the  ether  and  not  as  lines  having 
actual  existence  will  result  only  in  a  confusion  of  fact  and 
fancy  during  the  study  of  the  phenomena  of  magnetism. 

280.  Magnetic  Induction. — Place  a  pane  of  glass  on  a 
soft -iron  bar  about  2J  inches  long.  Sprinkle  iron-filings 
evenly  over  the  surface  and  tap  the  glass.  No  change 
occurs  suggesting  the  presence  of  magnetism  in  the  iron 
bar. 

Place  the  glass  over  the  north-seeking  end  of  a  bar 
magnet,  sprinkle  filings,  and  again  tap  the  glass.  The 
filings  now  appear  to  be  arranged  in  lines  which  radiate 
from  the  north-seeking  pole  of  the  magnet,  curve  around 
its  sides  and  then  enter  from  all  directions  at  the  south- 
seeking  pole.  It  is  this  appearance  that  first  suggested 
the  idea  of  lines  of  force.  The  lines  indicate  in  what  di- 
rection the  ether  in  the  field  around  the  magnet  is  strained. 

The  ether  within  the  body  of  the  magnet  is  also  strained. 
Lines  of  force  therefore  should  be  drawn  so  as  to  indicate 
both  the  direction  of  the  strain  of  the  ether  in  the  field 
around  the  magnet  and  also  of  the  ether  within  the  body 
of  the  magnet.  Lines  of  force  drawn  in  this  manner  ap- 
pear to  traverse  complete  circuits  (not  circles).  They 
may  be  imagined  to  pass  from  the  north- seeking  pole 
through  the  field  toward  the  south-seeking  pole,  and 
then  to  complete  their  circuits  by  passing  through  the 
body  of  the  magnet  from  the  south-seeking  pole  toward 
the  north-seeking  pole.  An  examination  of  the  lines  in- 


MAGNETISM   AND   ELECTRICITY  299 

dicated  by  the  iron-filings  will  show  that  poles  are  not 
definite  points  at  the  ends  of  magnets,  but  rather  indefinite 
regions,  near  the  ends,  from  which  the  lines  appear  to 
radiate,  or  toward  which  they  appear  to  converge. 

Lift  the  pane  of  glass  carefully,  so  as  not  to  disarrange 
the  filings.  Fold  a  piece  of  paper  neatly  over  the  north- 
seeking  end  of  the  magnet,  and  press  this  end  of  the 
magnet  against  a  soft-iron  bar.  The  paper  prevents 
actual  contact  between  the  magnet  and  the  soft  iron. 
Then  replace  the  glass,  lowering  it  carefully  so  that  the 
appearance  of  lines  of  force  radiating  from  the  north- 
seeking  pole  of  the  magnet  is  restored.  Again  tap  the  glass. 
The  filings  rearrange  themselves,  and  many  lines  of  force 
now  appear  to  radiate  from  the  sides  and  farther  end  of  the 
soft-iron  bar  as  well  as  from  the  end  of  the  magnet.  This 
indicates  that  the  ether  within  and  around  the  soft-iron 
bar  now  is  also  strained.  It  should  therefore  show  the 
same  magnetic  properties  as  the  magnet,  although  possi- 
bly in  a  lesser  degree.  This  may  be  readily  demonstrated 
by  placing  the  farther  end  of  the  soft-iron  bar  in  a  pile  of 
tacks  while  the  north-seeking  pole  of  the  bar  magnet  with 
its  paper  cover  is  still  held  against  the  other  end  of 
the  bar.  In  this  condition  the  soft-iron  bar  picks  up 
tacks. 

As  soon  as  the  magnet  is  withdrawn  from  the  soft- 
iron  bar,  the  tacks  drop  off.  After  the  removal  of  the 
magnet,  cover  the  soft-iron  bar  with  the  pane  of  glass, 
sprinkle  iron-filings,  and  tap  the  glass.  As  might  be  ex- 
pected from  the  dropping  off  of  the  tacks  when  the  mag- 
net is  removed,  no  appearance  of  lines  of  force  is  pro- 
duced around  the  soft  iron  when  not  in  the  immediate 
vicinity  of  the  magnet. 

The  magnet  is  said  to  induce  magnetism  into  the  soft- 


300 


ELEMENTARY  PHYSICS 


iron  bar.  Since  soft  iron  does  not  retain  magnetism  well, 
the  bar  is  only  temporarily  a  magnet.  It  should  scarcely 
be  necessary  to  state  that  the  strain  in  the  ether  as  shown 
by  the  iron-filings  is  not  due  to  the  tapping  of  the  glass. 
The  tapping  merely  assists  the  iron-filings  in  rearranging 
themselves  so  as  to  make  evident  any  magnetic  strain 
which  may  be  present  in  the  ether. 

281.  Magnetic  Permeability  of  Soft  Iron  and  Air. — It 
was  seen  in  the  preceding  experiment  that  the  filings  re- 
arranged themselves  after  the  soft-iron  bar  had  been 
added  to  the  magnet  and  the  glass  had  been  again  tapped. 


Hfl 


FIG.  120. 

This  indicates  that  the  presence  of  soft  iron  within 
the  field  of  a  magnet  changes  the  direction  of  the  strain 
of  the  ether  within  the  field.  This  may  also  be  shown  by 
other  experiments. 

Place  a  bar  of  soft  iron  a  short  distance  in  front  of  the 
poles  of  a  large  horseshoe  magnet.  Cover  bar  and  mag- 
net with  a  pane  of  glass  and  place  a  second  bar  of  soft  iron 
directly  over  the  first.  Sprinkle  iron-filings  evenly  over  the 
surface  and  tap  the  glass.  Notice  to  what  extent  the 
lines  formed  by  the  iron-filings  may  be  distinctly  traced 
beyond  the  soft-iron  bars  (Fig.  120).  Now  gently  raise 


MAGNETISM  AND   ELECTRICITY 


301 


the  glass,  remove  both  soft-iron  bars,  and  carefully  re- 
place the  glass  in  its  original  position  over  the  magnet. 
Tap  the  glass  again.  The  lines,  formed  by  the  filings, 
passing  from  the  north-seeking  to  the  south-seeking  pole, 
may  now  be  traced  distinctly  to  a  much  greater  distance 
in  front  of  the  magnet  (Fig.  121).  This  suggests  that  in 
the  first  case  the  soft-iron  bars  change  the  direction  of 
the  strain  in  the  ether  in  the  field  in  front  of  the  magnet, 
and  also  distinctly  weaken  the  strength  of  that  part  of 
the  field  which  lies  beyond  the  soft  iron.  Instead  of  a 
horseshoe  magnet,  two  bar  magnets  may  be  used.  Place 


.  121. 


the  magnets  side  by  side,  about  3  inches  apart,  with  the 
like  poles  pointing  in  opposite  directions.  Use  one  pair 
of  poles. 

This  principle  has  been  found  useful  to  a  certain  extent 
in  protecting  the  works  of  a  watch  from  magnetism.  If 
the  case  of  a  watch  be  constructed  of  soft  iron,  the  part  of 
the  field  enclosed  within  the  case  is  much  weaker  and  the 
works  are  in  a  measure  protected.  The  appearance  of  the 
watch  is  of  course  improved  by  plating  the  case  with 
gold. 

A  study  of  the  preceding  experiments  suggests  not  only 


302  ELEMENTARY   PHYSICS 

that  soft  iron  alters  the  direction  of  the  strain  in  the  ether 
in  the  field  surrounding  a  magnet,  but  that  it  appears  to 
take  up  a  large  part  of  the  strain  in  this  part  of  the  field, 
leaving  the  ether  beyond  in  a  less  strained  condition.  On 
this  account  soft  iron  is  said  to  be  more  permeable  to  mag- 
netism than  air. 

282.  Physical  Theory  of  Magnetization. — Magnetize  a 
straightened  piece  of  a  broken  watch-spring,  such  as  may 
be  obtained  from  any  jeweler.  Dip  the  entire  piece  un- 
der iron-filings;  on  its  removal  iron -filings  adhere  to  the 
ends  but  not  to  the  middle.  This  shows  that  for  the 
greater  part  of  the  length  of  the  magnetized  watch-spring 
the  strain  of  the  ether  lies  chiefly  within  the  body  of  the 
spring  and  is  practically  parallel  to  its  length.  At  the 
ends  of  the  spring  where  the  filings  adhere  the  direction 
of  this  strain  changes.  When  studied  in  connection  with 
iron-filings  the  appearance  is  as  if  lines  of  force  pass  out 
and  enter  chiefly  at  the  ends  of  the  magnet. 

With  a  wire  nipper,  cut  the  straightened  spring  at  a 
point  where  no  filings  adhere.  On  dipping  in  iron-filings, 
filings  are  found  to  adhere  at  the  cut  end.  The  lines  of 
force  now  appear  to  come  out  at  a  part  of  the  mag- 
netized spring  where  a  moment  before  no  magnetism  was 
shown.  This  indicates  a  change  in  the  direction  of  the 
strain  at  the  point  where  the  spring  was  cut.  Continue 
the  cutting,  and  even  the  smallest  fragment  of  watch- 
spring  will  show  magnetism  at  each  end.  In  each  case 
the  part  remaining  appears  to  be  a  complete  magnet, 
but  smaller  and  weaker.  It  is  possible  to  conceive  that 
this  cutting  might  be  continued  indefinitely  and  that  if  a 
single  molecule  of  iron  could  be  secured,  each  end  of  this 
molecule  would  show  magnetic  properties,  one  end  being 
north-seeking  and  the  other  end  south-seeking. 


MAGNETISM   AND   ELECTRICITY  303 

In  accordance  with  this  view  a  magnet  consists  of  a 
combination  of  many  magnets,  each  molecule  being*  a 
tiny  magnet.  If  the  north-seeking  ends  of  several  mag- 
nets are  held  together,  their  combined  effect  is  greater 
than  that  of  any  one  of  the  magnets.  In  the  same  man- 
ner, many  millions  of  molecules  of  iron  with  their  north- 
seeking  poles  all  directed  in  the  same  direction  ought  to 
show  considerable  magnetism  (Fig.  122,  A).  If  these 
molecules  became  considerably  disarranged,  the  north- 


S 


FIG.  122. 

seeking  poles  and  the  south-seeking  poles  might  coun- 
teract each  other  (Fig.  122,  B).  This  principle  may  be 
illustrated  by  laying  the  north-seeking  pole  of  one  mag- 
net upon  the  south-seeking  pole  of  another  and  then 
trying  to  lift  up  tacks  while  these  ends  are  in  contact 
with  one  another.  It  will  be  found  that  in  this  condition 
the  magnets  will  lift  up  less  tacks  than  if  either  magnet 
had  been  used  alone.  In  fact,  it  frequently  happens  that 
when  the  north-seeking  and  south-seeking  poles  are  used 
together,  scarcely  any  tacks  can  be  lifted. 


304 


ELEMENTARY   PHYSICS 


According-  to  this  theory,  the  magnetizing-  of  an  iron  or 
steel  bar  consists  simply  in  drawing  the  corresponding 
ends  of  all  the  molecules  in  the  same  direction  by  strok- 
ing the  bars  of  which  they  are.  a  part  by  means  of  some 
strong  magnet,  always  in  the  same  direction.  The  like 
poles  of  the  molecules  are  thus  caused  to  face  in  the 
same  direction.  In  this  position  they  act  together  as  a 
unit.  Twisting  the  magnetized  bar,  pounding  it,  or  heat- 
ing it,  considerably  disarranges  the  molecules.  The 
poles  of  the  molecules  now  face  in  so  many  directions 
that  they  counteract  each  other,  and  the  bar  no  longer 
shows  pronounced  magnetic  effects.  The  bar  is  said  to 
be  demagnetized. 


STATIC  ELECTRICITY 

283.  Electrification  by  Rubbing.— Suspend  a  pith  ball,  a 
quarter  of  an  inch  in  diameter,  at  the  end  of  fine  silk 

thread  about  ten  inches  long.     Hold  the 
end  of  an  ebonite  rod  near  the  pith  ball. 
The  pith  ball  is  not  affected  in  any  man- 
ner by  the  presence  of  the  ebonite  rod. 
Now  rub  the  end  of  the  ebonite  rod  with 
flannel  and  bring  it  near  the  pith  ball. 
The  pith  ball  is  at  first  attracted,  but, 
after  contact  has  been  broken,  it  is  re- 
pelled (Fig.  123).    Eubbing  has 
given    the    ebonite    rod    some 
properties  which  it  did  not  pos- 
sess before.     The  rod  is  said  to 
be  electrified. 

284.  Electrification  by  Contact  with  an  Electrified  Body.— 
Suspend  two  pith  balls  at  the  same  level  by  means  of 


FIG.  123. 


MAGNETISM   AND   ELECTRICITY  305 

two  silk  threads.  Bring-  the  upper  ends  of  the  threads 
in  contact  with  each  other.  The  pith  balls  strike  against 
one  another  and  show  no  unusual  properties.  Now  bring 
a  strongly  electrified  ebonite  rod  into  contact  with  the 
pith  balls.  Withdraw  the  rod  and  again  hold  together 
the  upper  ends  of  the  silk  threads.  The  pith  balls  re- 
main apart,  showing  that  they  repel  each  other.  The 
pith  balls,  in  this  case,  have  become  electrified,  not  by 
rubbing,  but  by  contact  with  a  body  which  was  elec- 
trified. In  this  condition,  the  pith  ball  is  often  said  to 
be  charged  with  electricity,  and  the  experiment  indicates 
that  two  bodies  electrified  in  t/ie  same  manner  repel  each 
other. 

285.  Conductors. — Suspend  two  pith  balls  from  a  glass 
support  by  silk  threads  which  are  tied  together  at  the 
top,  thus  bringing  the  pith  balls  at  the  lower  ends  of  the 
threads  in  contact  with  one  another.     If  the  silk  threads 
are  touched  at  their  upper  ends  with  a  strongly  electri- 
fied ebonite  rod,  the  pith  balls  remain  in  contact.    If  the 
experiment  be  repeated  with  balls  suspended  by  cotton 
thread,  the  pith  balls  repel  each  other  strongly.     This 
indicates  that  cotton  conducts  electricity  fairly  well  from 
the  electrified  ebonite  rod  to  the  pith  balls,  but  silk  is 
evidently  a  very  poor  conductor. 

286.  The  Electroscope. — A  pair  of  pith  balls  suspended 
at  the  lower  ends  of  cotton  threads,  tied  together  at  the 
top,  may  be  used  to  detect  the  presence  of  electricity 
in  a  body.     The  mutual  repulsion  of  the  pith  balls  on 
bringing  any  object  in  contact  with  the  cotton  threads  at 
once  indicates  that  the  object  is  electrified.     There  are, 
however,  much  better  conductors  than  cotton,  and  much 
lighter  bodies  than  cotton  thread  and  pith  balls.     By 
means  of  these  bodies   it  is  possible  to  detect   much 


306 


ELEMENTARY  PHYSICS 


lighter  charges  of  electricity  than  by  means  of  the  thread 
and  pith  balls.  All  metals  are  better  conductors  of  elec- 
tricity than  cotton.  Several  of  these  metals  can  be  se- 
cured in  the  form  of  exceedingly  thin  sheets.  Tin,  alum- 
inum, and  gold  foil  are  examples  of  metals  in  this  form. 
When  sufficiently  thin,  sheets  of  these  metals  are  so  light 
that  they  move  at  the  slightest  breath  of  air.  If  two  thin 
strips  of  foil  are  fastened  at  the  lower  end  of  a  metal 
rod,  and  an  electrified  body  is  brought  into  contact 
with  the  supporting  rod,  the  electricity  spreads  readily 
throughout  the  rod  and  a  part 

passes    into    the    two  .^ —          'A        strips  of 

foil    which    at    once      //  .^^ "      V      repel  each 

other.  The 
strips  of 

foil  are  so  light,  that  bodies,  which  are  so 
weakly  electrified  that  contact  with  pith 
balls  produces  no  perceptible  repulsion, 
will,  when  brought  in  contact  with  the 
supporting  rod,  cause  the  strips  of  foil  to 
repel  each  other  violently. 

This  principle  is  used  in  the  construc- 
tion of  an  instrument  for  the  detection  and 
study  of  charges  of  electricity,  called  the 
electroscope  (Fig.  124).     SLis  consists  of  a 
brass  rod  with  a  knob  "the  top,  passing 
through   a  rubber    stopper    into  a  flask. 
The  lower  end  is  broadened  and  serves  as 
the  place  of  attachment  of  two  thin  strips 
FIG.  124.          of  gold  leaf      If  an  electrified  ebonite  rod 
is  brought  in  contact  with  the    knob,   the    electricity 
passes  down  the  brass  rod  into   the  gold  leaves,  and 
consequently  the  gold  leaves  repel  each  other. 


MAGNETISM  AND   ELECTRICITY 


307 


287.  Electricity  Passes  from  Points  of  Strong  Electrifica- 
tion to  Points  of  Less  or  No  Electrification. — If  a  slightly 
electrified  ebonite  rod  is  brought  in  contact  with  the 
electroscope,  the  gold  leaves  diverge  slightly.  If  a 
more  strongly  electrified  rod  is  used,  the  gold  leaves 
diverge  more.  If  an  electroscope  with  strongly  diverg- 
ing gold  leaves  is  brought  in  contact  with  an  unelectri- 
fied  electroscope,  by  means  of  a  long  copper  wire,  the 
leaves  of  the  second  electroscope  diverge.  If  by  the 


FIG.  125. 

same  means  a  strongly  electrified  electroscope  is  brought 
in  contact  with  a  moderately  electrified  electroscope,  the 
strongly  diverging  leaves  of  the  first  electroscope  col- 
lapse partly,  while  those  in  the  second  instrument  di- 
verge more  (Fig.  125). 

In  performing  theso  experiments,  the  copper  wire  must 
not  be  allowed  to  come  in  contact  with  the  table,  the 
hand,  or  any  other  material  which  will  conduct  electric- 
ity. It  may  be  handled  most  readily  by  twisting  the 
wire  around  glass  rods  which  may  be  held  in  the  hand. 


308  ELEMENTAEY   PHYSICS 

These  experiments  show  that  electricity  passes  from 
points  of  strong  electrification  to  points  of  less  or  even 
of  no  electrification.  Whatever  may  be  the  true  nature  of 
electricity,  this  transfer  of  electricity  must  be  due  to 
some  force  or  to  a  relative  inequality  of  forces  residing 
in  different  objects.  In  the  last  experiment  both  the 
electricity  in  the  strongly  electrified  electroscope  and 
that  in  the  less  electrified  instrument  tend  to  move  away 
from  these  objects  to  points  of  still  weaker  electrification. 
However,  when  the  two  are  brought  in  contact  with  one 
another  by  means  of  the  copper  wire,  the  electricity  ap- 
pears to  move  from  the  more  electrified  to  the  less  elec- 
trified body,  and  the  force  with  which  it  moves  must 
unquestionably  depend  upon  the  difference  of  the  two 
forces  urging  the  electricity  in  the  two  electroscopes 
toward  some  other  point. 

288.  Potential,   Electromotive    Force. — The    force   with 
which  the  electricity  in  each  electroscope  tends  to  move 
to  some  other  point,  is  called  the  potential  of  the  electric- 
ity in  that  instrument.     The  force  with  which  a  part  of 
the  electricity  in  the  more  highly  charged  instrument  is 
urged  to  move  from  this  electroscope  to  the  one  less 
strongly  charged,  is  due  to  the  difference  of  potential  be- 
tween these  instruments.     The  force  with  which  a  part  of 
the  electricity  moves  in  consequence  of  this  difference  of 
potential  is  often  spoken  of  as  though  it  were  a  single 
separate  force,  instead  of  an  effect  due  to  the  difference  in 
strength  of  two  forces,  and  is  then  called  the  electricity 
moving  force,  or  the  electromotive  force. 

289.  Electrification  Caused  by  Contact  of  Two   Different 
Substances. — In  the  preceding  experiments,  ebonite  was 
electrified  by  rubbing  it  with  flannel.     Many  other  bod- 
ies may  be  electrified  in  a  similar  manner.     Glass  is  fre- 


MAGNETISM  AND   ELECTRICITY  309 

quently  electrified  by  rubbing  it  with  silk.  In  fact,  the 
rubbing  together  of  any  unlike  substances  at  once  results 
in  electrification.  Bub  the  coat  sleeve  vigorously  over  a 
piece  of  paper  and  place  the  paper  against  the  wall.  It 
adheres  to  the  wall,  showing  that  it  has  become  electri- 
fied. Rub  another  piece  over  the  varnished  surface  of  a 
school-room  desk,  and  the  paper  will  show  less  tendency 
to  slide  down  the  inclined  surface  than  it  exhibited  be- 
fore it  was  rubbed.  From  this  it  might  appear  that  rub- 
bing is  essential  to  the  electrification  of  objects. 

It  is  becoming  evident,  however,  from  more  recent 
observations,  that  the  essential  element  in  electrification 
is  mere  contact  between  unlike  bodies.  Mere  contact 
between  ebonite  and  flannel  produces  a  state  of  electrifi- 
cation, but,  since  both  ebonite  and  flannel  are  very  poor 
conductors  of  electricity,  it  is  difficult  for  electricity  to 
spread  from  the  points  touched  to  other  parts,  and  the 
state  of  electrification  is  therefore  practically  confined 
to  points  of  actual  contact.  Rubbing  brings  many  more 
points  on  the  ebonite  rod  in  actual  contact  with  many 
parts  of  the  flannel,  and  hence  the  degree  of  electrifica- 
tion of  the  rod  is  very  much  increased  by  rubbing. 

Moreover,  since  ebonite  is  a  non-conductor,  only  the 
end  rubbed  shows  evidence  of  electrification ;  but  if 
one  end  of  a  good  conductor,  such  as  a  brass  rod,  is 
rubbed,  the  entire  rod  gives  evidence  of  electrification. 
In  order  to  prevent  the  escape  of  electricity  through  the 
hand,  the  brass  rod  must,  of  course,  be  held  by  means  of 
some  non-conducting  material — for  instance,  at  the  end  of 
a  glass  rod,  or  by  means  of  a  piece  of  silk. 


310  ELEMENTARY   PHYSICS 

CURRENT   ELECTRICITY 

290.  A  Continuous  Current  of  Electricity  Maintained  by 
Chemical  Action. — Whenever  any  solid  is  brought  in  con- 
tact with  any  liquid,  for  instance,  by  dipping-  the  solid 
in  the  liquid,  the  solid  is  electrified.  The  evidence  of 
electrification  may  be  made  especially  clear  if  both  solid 
and  liquid  are  good  conductors.  When  any  two  solid 
conductors  are  dipped  rnto  the  same  liquid  conductor, 
they  will,  as  a  rule,  be  found  to  differ  in  their  degree  of 
electrification.  Hence,  the  charges  of  electricity  in  the 
two  solids  will  be  at  a  different  potential,  or  will  differ  in 
the  force  with  which  they  tend  to  move  to  other  points. 
In  consequence  of  this  difference  of  potential,  the  elec- 
tricity tends  to  move  from  the  point  of  higher  potential 
to  that  at  lower  potential,  as  soon  as  the  solid  conductors 
are  joined  by  means  of  wire  which  is  also  a  conductor. 
The  result  is  a  current  of  electricity. 

It  does  not  differ  essentially  from  the  current  of  elec- 
tricity produced  by  two  electroscopes  electrified  to  dif- 
ferent potentials,  connected  by  means  of  a  conducting 
wire. 

When  a  strongly  electrified  electroscope  is  connected 
with  a  weakly  electrified  electroscope,  the  current  of  elec- 
tricity lasts  only  an  instant  while  the  potentials  in  the 
two  electroscopes  are  being  equalized.  This  takes  place 
so  quickly  that  the  current  is  only  momentary.  When, 
on  the  contrary,  two  different  solid  conductors  are 
dipped  in  a  liquid  conductor  which  attacks  at  least  one 
of  them  chemically,  the  chemical  action  keeps  up  a 
difference  of  potential  in  the  two  solids,  even  after  they 
have  been  connected  a  long  time  by  the  wire.  It  may 
be  that  chemical  action  results  in  a  continual  renewal 


MAGNETISM  AND   ELECTRICITY 


311 


of  contact  between  the  solids  and  the  molecules  of  the 
liquid,  so  that  the  inequality  of  the  charges  of  elec- 
tricity in  the  solids  is  renewed  as  rapidly  as  these 
charges  are  equalized  by  the  connecting-  wire.  In  other 
words,  owing  to  chemical  action,  there  is  a  continuous 
current  of  electricity  through  the  wire. 

291.  The  Electric  Cell. — When  copper  and  zinc  are 
dipped  into  dilute  sulphuric  acid,  both  copper  and  zinc 
are  electrified  ;  the  potential  of  copper, 
where  in  contact  with  the  air,  is  higher 
than  the  potential  of  that  part  of  the  zinc  ' 
which  extends  up  into  the  air.  Hence 
a  current  of  electricity  flows  from  the 
copper  through  the  wire  to  the  zinc,  as 
soon  as  these  metals  are  connected  by 
means  of  wire  (Fig.  126). 

The  zinc  is  attacked  by  the  sulphuric 
acid.  It  continues  to  be  attacked  even 
after  the  flow  of  electricity  through  tho 
connecting  wire  has  been  started.  In 
consequence  of  this  continued  chem- 
ical action,  the  difference  of  potential 
between  the  copper  and  zinc  is  kept  up,  and  the  flow  of 
electricity  continues.  The  zinc  gradually  wears  away 
and  must  be  replaced  from  time  to  time.  The  sulphuric 
acid  solution  must  also  be  replenished. 

Instead  of  copper,  carbon  is  frequently  used,  and  in- 
stead of  sulphuric  acid,  sal  ammoniac,  copper  sulphate, 
or  a  mixture  of  sulphuric  acid  and  potassium  bichro- 
mate is  often  employed.  The  combination  of  copper  and 
zinc,  or  of  copper  and  carbon  plates  with  the  chemical 
solution,  including  the  jar  containing  the  solution,  is 
usually  called  an  electric  cell  or  battery. 


FIG.  126. 


312 


ELEMENTARY  PHYSICS 


The  sal-ammoniac  cell  is  the  cell  most  frequently  used 
for  electric  bells  and  buzzers.  It  consists  of  zinc  and 
carbon  plates,  rods,  or  cylinders  dipped  into  a  solution  of 
sal  ammoniac. 

In  the  cell  commonly  used  for  telegraphic  instruments, 
the  gravity  cell  (Fig.  127),  the  metals  employed  are  zinc  and 
copper.  The  copper,  in  the  form  of  thin  sheets,  is  placed 
in  the  bottom  of  the  cell,  and  the  zinc  is  suspended  in  the 
upper  part  of  the  liquid.  When  the  zinc  is  cast  into  the 
form  of  diverging  bars,  it  slightly  resembles  the  spread- 
ing toes  of  some  birds,  and  in  that  case  the  gravity  cell 
is  often  called  a  crowfoot  battery.  Crystals  of  copper 
sulphate  are  thrown  into  the  water  and  dissolve.  After 
the  cell  has  been  used  a  short  time,  zinc  sulphate  is 
formed.  The  solution  of  zinc  sulphate  is  lighter  than 
that  of  copper  sulphate,  and  therefore  rises  to  the  top  of 

the  cell.  The  contrast  be- 
tween the  colorless  zinc  sul- 
phate solution  and  the  blue 
copper  sulphate  solution  be- 
neath is  very  distinct  in  a  cell 
which  has  been  in  use  for 
several  days.  The  zinc  sul- 
phate is  in  contact  with  the 
zinc  plate.  The  copper  sul- 
phate is  in  contact  with  the 
sheets  of  copper.  The  chem- 
ical action  is  complicated. 
The  zinc  wears  away.  The 
copper  in  the  copper  sulphate 
becomes  separated  from  the 

copper  sulphate  and  settles  on  the  copper  plate.     The 
copper  plate,  therefore,  increases  in  thickness,  but  the 


MAGNETISM   AND   ELECTRICITY  313 

copper  sulphate  in  the  solution  must  be  replenished 
from  time  to  time  by  the  addition  of  more  copper  sul- 
phate crystals. 

The  potassium  bichromate  cell  consists  of  carbon  and 
zinc  plates  placed  in  a  solution  consisting-  of  sulphuric 
acid,  potassium  bichromate,  and  water. 

The  plate  having  the  higher  potential,  usually  copper 
or  carbon,  is  called  the  positive  plate.  The  plate  having 
the  lower  potential,  almost  invariably  zinc,  is  called  the 
negative  plate.  For  the  purpose  of  connecting  wires  to 
these  plates,  binding  posts  are  added.  The  current 
starts  011  that  part  of  the  copper  or  carbon  plate  which 
is  out  of  the  liquid,  passes  through  the  wire  to  the 
zinc  plate,  and  then  returns  through  the  liquid  to  the 
copper  or  carbon  plate. 

ELECTROMAGNETISM 

292.  Compass  Needle  Affected  by  a  Wire  Bearing  an  Elec- 
tric Current. — Connect  the  carbon  plates  of  four  potas- 
sium bichromate  cells  to  one  another  by  means  of  wires. 
Connect  in  the  same  manner  the  four  zinc  plates.  Any 
wire  connecting  one  of  these  carbon  plates  with  one  of 
the  zinc  plates  will  now  conduct  the  electricity  from  all 
of  the  carbon  plates  to  the  zinc  plates.  The  four  cells 
now  act  like  one  huge  cell.  Cells  arranged  in  this  man- 
ner are  said  to  be  connected  in  parallel.  Use  a  connect- 
ing wire  about  five  feet  long,  and  fasten  a  length  of 
about  one  foot  in  such  a  manner  that  the  current  will 
pass  vertically  upward.  Bend  the  remainder  of  the  wire 
so  as  to  keep  it  as  far  distant  from  this  length  as  pos- 
sible. 

The  plates  of  potassium  bichromate  cells  are  usually 


314 


ELEMENTARY   PHYSICS 


so  arranged  that  all  the  plates  can  be  raised  at  the  same 
time  from  the  jar  (Fig.  128).  The  plates  are  raised  to  pre- 
vent the  liquid  from  attacking  the  zinc  plates  when  the 
cells  are  not  in  use.  While  in  this  condition  no  current 
can  flow  through  the  wire.  Hold  a  small  compass,  not 
more  than  an  inch  in  diameter,  on  various  sides  of  the 


FIG.  128. 

vertical  part  of  the  wire.  In  all  positions  of  the  com- 
pass the  needle  points  northward.  Lower  the  plates 
into  the  liquid  and  the  needle  now  points  southward  on 
the  west  side  of  the  wire,  eastward  on  the  south  side, 
northward  on  the  west  side  and  westward  on  the  north 
side.  If  intermediate  points  be  tried,  the  needle  will  be 
found  to  point  in  intermediate  directions.  In  conse- 


MAGNETISM  AND   ELECTRICITY  315 

quence  of  the  electric  current  some  force  evidently  urges 
the  needle  of  the  compass  to  occupy  positions  at  variance 
with  its  usual  position  when  held  at  a  distance  from 
objects  conducting-  electric  currents. 

293.  A  Magnetic  Field  Encircles  a  Wire  Bearing  an  Elec- 
tric Current. — It  has  already  been  shown  that  bar  magnets 
are  able  to  influence  the  direction  of  the  compass  needle 
(§  277).  It  is  believed  that  this  influence  is  due  to  a 
strain  in  the  ether  in  the  field  surrounding  the  magnet. 
The  direction  of  strain  of  the  ether  in  a  magnetic  field 
may  be  indicated  by  lines  called  lines  of  force.  In  the 
field  of  a  magnet  the  strain  is  represented  by  lines  leav- 
ing in  all  directions  at  one  pole,  curving  around  through 
the  air,  entering  from  all  sides  at  the  other  pole,  and  trav- 
ersing the  length  of  the  magnet,  thus  forming  complete 
circuits. 

The  experiment  with  the  compass  needle  held  near  a 
wire  carrying  a  current  of  electricity  (§  292)  suggests  that 
the  ether  is  strained  also  in  the  field  surrounding  a  wire 
while  an  electric  current  is  passing.  The  different  posi- 
tions occupied  by  the  compass  needle  indicate  that  the 
lines  of  force  representing  this  strain  must  be  drawn  in 
the  form  of  circles  surrounding  the 
wire.  This  may  be  shown  also  by 
means  of  iron-filings,  sprinkled  on  a 
sheet  of  card-board.  If  a  wire  is  passed 
vertically  through  the  card-board  and  a 
heavy  current  is  turned  on,  a  slight 
jarring  of  the  card-board  causes  the 
filings  to  arrange  themselves  in  circles  FlG.  129. 

surrounding  the  wire  (Fig.  129).     No 
part  of  the  path  of  these  lines  of  force  passes  through 
the  wire  ;    nothing  corresponding  to  poles  exists,  and 


816  ELEMENTAEY  PHYSICS 

hence  there  is  no  radiation  of  lines  of  force  as  at  the 
poles  of  magnets.  If  a  very  small  compass  needle  is  held 
at  various  distances  from  the  wire,  it  becomes  evident 
that  the  magnetic  influence  of  the  current  is  greatest  in 
the  immediate  vicinity  of  the  wire,  and  becomes  rapidly 
less  as  the  distance  from  the  wire  increases.  This  may 
be  represented  by  diminishing  the  number  of  lines  of 
force  as  the  distance  from  the  wire  increases. 

The  direction  assumed  by  a  compass  needle  in  the  field 
of  a  magnet  may  be  determined  mechanically  by  the  rule 
that  the  needle  points  forward  at  every  part  of  any  line  of 
force  if  followed  from  the  north-seeking  to  the  south- 
seeking  pole. 

There  is  also  a  convenient  mechanical  rule  for  deter- 
mining the  direction  assumed  by  a  needle  placed  in  the 
magnetic  field  of  a  wire  carrying  a  current.  If  the  wire 
is  grasped  by  the  right  hand  so  that  the  fingers  encircle 
the  wire  and  so  that  the  thumb  points  in  the  direction 
in  which  the  current  is  flowing,  the  tips  of  the  fingers  will 
point  in  the  direction  in  which  the  north-seeking  pole  of 
the  needle  will  face  if  placed  on  the  same  side  of  the 
wire.  From  this  thumb  and  finger  rule  it  is  at  once  evi- 
dent that  if  a  current  is  sent  downward  through  a  vertical 
wire,  the  lines  of  force  must  travel  around  the  wire  in  a 
direction  exactly  opposite  to  that  followed  when  the  cur- 
rent is  sent  upward  through  the  same  wire.  Hence  all 
the  positions  of  the  compass  will  be  reversed.  Show 
this  by  actual  experiment. 

294.  Direction  of  Current  Determined  by  Means  of  the 
Compass. — If  a  wire  is  held  in  a  horizontal  position  directly 
above  a  needle  so  that  the  current  passes  northward  over 
the  needle,  the  thumb  and  finger  rule  given  in  the  pre- 
ceding paragraph  indicates  that  the  north-seeking  end  of 


MAGNETISM   AND    ELECTRICITY 


31 


the  needle  must  turn  westward.  If  the  current  is  sent 
southward  over  the  needle,  the  north-seeking  end  of  the 
needle  turns  eastward. 

Determine  in  what  direction  the  needle  will  point  if  you 
send  the  current  northward  under  the  needle,  or  if  you 
send  it  southward  beneath  the  same.  In  what  direction 
do  the  lines  of  force  pass  011  the  side  of  the  wire  when  the 
current  passes  northward?  When  the  current  passes 
southward  ? 

295,  A  Loop  through  which  an  Electric  Current  is  Passing 
Behaves  like  a  Magnet.— Bend  the  wire  connected  with  the 
potassium  bichromate  cells 
in  such  a  manner  that  a  part 
of  it  forms  a  single  horizon- 
tal loop,  and  so  that  the  cur- 
rent goes  westward  along  the 
northern  curve  of  the  loop 
(Fig.  130).  Making  use  of 
the  thumb  and  finger  rule, 
and  representing  the  direc- 
tion of  the  strain  in  the  ether 
around  the  wire  by  means  of  lines  of  force,  it  will  be 
seen  that  the  lines  of  force  come  upward  at  all  points 
along  the  interior  of  the  loop,  curve  around  the  top  of 
the  wire,  pass  downward  along  the  outer  part  of  the  loop, 
and  re-enter  at  the  lower  face.  In  this  form  of  represen- 
tation the  lines  of  force  leave  the  upper  face  of  the  loop 
and  radiate  in  all  directions  just  as  they  leave  the  north- 
seeking  pole  of  a  magnet.  At  the  lower  face  of  the  loop 
the  lines  of  force  re-enter,  as  is  true  also  of  the  lines  of 
force  at  the  south-seeking  end  of  the  magnet.  In  both 
cases  the  lines  of  force  indicate  merely  the  direction  of 
the  strain  in  the  ether,  and  the  direction  toward  which 


FIG.  130. 


318 


ELEMENTARY   PHYSICS 


the  north-seeking-  end  of  the  compass  needle  within  the 
field  points.  Therefore  the  character  of  the  strain  of  the 
ether  on  the  upper  face  of  the  loop  is  similar  to  the  strain 
at  the  north-seeking-  pole  of  a  magnet.  From  this  it 
might  be  expected  that  the  upper  face  of  the  loop  ought 

to  possess  magnetic  prop- 
erties similar  to  the  north  - 
seeking  pole  of  a  magnet, 
while  the  lower  face  should 
possess  properties  similar 
to  those  of  a  south-seeking 
pole. 

If  the  current  of  electrici- 
ty is  sent  in  an  opposite  di- 
rection around  the  loop,  or 
if  the  wire  is  coiled  so  that 
the  current  passes  in  an  op- 
posite direction,  the  upper 
face  of  the  loop  becomes 
south-seeking,  and  the  low- 
er north-seeking. 

296.  The  Electromagnet. — 
Bend  a  wire  so  as  to  pro- 
duce a  succession  of  hori- 
zontal loops  forming  a  long, 
close  coil.  If  the  ends  of 
the  wire  be  connected  with 
the  potassium  bichromate 

cells  so  that  the  current  goes  westward  along  the  north- 
ern curve  of  the  loops,  the  upper  face  of  each  loop  will 
tend  to  be  north-seeking  while  the  lower  face  will  be 
south -seeking  (Fig.  131).  The  combined  effect  of  the 
different  loops  is  such  as  to  cause  a  considerable  in- 


FlG.  131. 


MAGNETISM   AND   ELECTRICITY  319 

crease  in  the  strain  of  the  ether,  especially  along*  the  in- 
terior of  the  coil.  This  is  represented  diagrainmatically 
by  drawing-  the  lines  of  force  in  such  a  manner  that  many 
of  the  lines  of  force  pass  from  loop  to  loop  lengthwise 
through  the  centre  of  the  coil,  thus  increasing  the  total 
number  of  the  lines  of  force  passing-  upward  throug-h  the 
centre  of  the  individual  loops. 

We  have  already  learned  that  soft  iron  is  much  more 
permeable  to  magnetism  than  air.  The  ether  within  the 
soft  iron  appears  to  be  strained  more  readily  than  the 
ether  in  the  surrounding  air.  The  result  is  that,  if  a  bar 
of  soft  iron  is  placed  lengthwise  along  the  centre  of  the 
coil,  the  strain  of  the  ether  along  the  interior  of  the  coil 
is  much  increased,  and  the  magnetic  effects  exhibited  at 
the  ends  where  the  soft  iron  projects  are  also  greatly 
strengthened.  This  may  be  represented  diagrammatical- 
ly  by  increasing  the  number  of  lines  of  force  traversing 
the  length  of  the  coil  when  soft  iron  is  placed  along  its 
interior.  The  combination  of  the  coil  with  the  soft- 
iron  core  shows,  therefore,  far  greater  magnetic  proper- 
ties at  the  end  of  the  coil  than  when  the  coil  is  used 
alone. 

The  end  of  the  coil  or  of  the  soft  iron  within  the  coil 
from  which  the  lines  of  force  are  represented  as  coming 
out,  acts  like  a  north-seeking  pole  of  a  bar  magnet.  The 
opposite  end  at  which  the  lines  of  force  are  represented 
as  coming  in,  acts  like  a  south-seeking  pole. 

A  modification  of  the  thumb  and  finger  rule  will  enable 
any  one  quickly  to  determine  the  magnetic  properties  of 
the  poles  of  any  coil  through  which  a  current  is  passing. 
Place  the  thumb  along  one  of  the  loops  of  the  coil  in  such 
a  manner  as  to  point  in  the  direction  in  which  the  cur- 
rent is  passing.  Imagine  that  the  fingers  grasp  the  wire 


320  ELEMENTARY   PHYSICS 

of  this  loop,  but  that  the  tips  of  the  fingers  lie  within  the 
centre  of  the  coil.  The  end  of  the  coil  toward  which  the 
finger  tips  would  point  is  the  north-seeking  end  of  the 
coil.  This  may  be  easily  tested  by  means  of  a  compass 
needle,  whose  north-seeking  pole  should  be  repelled  by 
this  end  of  the  coil  as  long  as  a  current  of  electricity  is 
passing. 

It  should  be  noted  that  the  strain  in  the  ether  in  the 
field  around  a  wire  is  dependent  entirely  upon  the  pres- 
ence of  a  current  in  the  wire.  When  there  is  no  current, 
the  ether  is  not  strained.  The  coil  containing  the  soft 
iron  is  therefore  a  magnet  only  while  a  current  of  elec- 
tricity is  passing  through  the  coil.  For  this  reason  the 
combination  of  the  coil  with  its  soft-iron  core  is  called  an 
electromagnet. 

If  the  coil  consists  of  many  loops  and  if  a  sufficient 
current  of  electricity  is  passed  through  the  coil,  an  elec- 
tromagnet can  be  made  much  more  powerfully  magnetic 
than  any  permanent  steel  magnet  of  the  same  weight. 
Electromagnets  are  employed  for  many  instruments 
(electric  bells,  telegraph  instruments)  in  which  it  is  nec- 
essary that  the  magnets  employed  should  be  magnetic 
only  temporarily.  In  these  cases,  permanent  magnets 
would  be  useless. 

Electromagnets  are  usually  made  in  the  form  of  two 
short  coils,  traversed  lengthwise  by  pieces  of  so'ft  iron. 
These  pieces  of  soft  iron  are  connected  at  one  end  by  a 
transverse  piece,  thus  having  a  slight  resemblance  to  a 
permanent  horseshoe  magnet.  The  object  to  be  at- 
tracted usually  consists  of  a  fourth  piece  of  soft  iron  held 
a  short  distance  away  from  the  poles  of  the  electromag- 
net by  a  spring.  This  movable  piece  of  soft  iron  is 
called  the  armature.  Most  of  the  lines  of  force  from  the 


MAGNETISM  AND   ELECTRICITY 


321 


north-seeking  pole  of  the  electromagnet  pass  through 
the  air  to  the  nearest  part  of  the  armature,  through  the 
armature,  thence  from  the  other  end  of  the  armature 
through  the  air  to  the  south-seeking  pole  of  the  electro- 
magnet, and  then  return  through  the  soft  iron  to  the 
north-seeking  pole. 

As  soon  as  a  current  of  electricity  is  passed  through 
the  coils  of  the  electromagnet,  the  armature  is  attracted. 
The  instant  the  current  of  electricity  is  broken,  the  elec- 


.  132. 


tromagnet  loses  its  magnetism  and  the  spring  attached 
to  the  armature  draws  the  armature  away. 

297.  The  Sounder  Used  in  Telegraphy, — In  the  sounder, 
an  instrument  used  in  telegraphy  (Fig.  132),  the  poles, 
N  S,  of  the  electromagnet  face  upward  and  the  arma- 
ture, A,  attached  to  a  short  horizontal  brass  tube,  is 
placed  directly  over  the  poles.  Ordinarily  the  armature 
is  kept  at  a  short  distance  from  the  electromagnet  by 
a  spring  attached  to  the  farther  end  of  the  brass  tube. 
As  soon  as  a  current  of  electricity  is  passed  through  the 
electromagnet,  the  armature  is  drawn  down  and  the  end 


ELEMENTAEY   PHYSICS 

of  the  brass  tube  to  which  the  armature  is  attached  strikes 
against  a  vertical  post,  P,  and  the  instant  the  current  of 
electricity  ceases,  this  end  of  the  brass  tube  is  drawn  up 
by  the  spring  at  the  opposite  end  already  mentioned, 
and  strikes  against  the  lower  side  of  a  brass  arch,  C. 
The  clicks  against  the  vertical  pillar  and  those  against 
the  arch  differ  so  much  in  the  character  of  the  sound  that 
they  may  be  easily  distinguished.  By  means  of  these 
sounds,  it  may  be  readily  determined  how  often  and  for 
what  length  of  time  the  current  of  electricity  has  been 
passed  through  the  electromagnet. 

In  telegraphy,  a  current  of  electricity  sent  through  the 
electromagnet  for  a  short  time  is  represented  by  a  dot. 
One  sent  through  for  a  longer  time  is  represented  by  a 
dash.  And  one  sent  through  for  a  still  longer  length  of 
time  is  represented  by  a  dash  twice  or  three  times  as 
long.  The  various  letters  of  the  alphabet,  the  Arabic 
numerals,  and  the  signs  used  in  punctuation,  are  repre- 
sented by  a  combination  of  such  dots  and  dashes.  Any 
one  expert  enough  to  recognize  from  the  various  clicks 
how  often  and  for  what  length  of  time  the  current  of 
electricity  was  sent  through  the  electromagnet  of  the 
sounder,  can  easily  determine  what  letters,  figures, 
and  signs  it  is  desired  to  convey  by  means  of  the  tele- 
graph. 

The  following  table  represents  the  system  in  ordinary 
use : 


B 

«*•  •§•• 

j 


A  ,.f. 


W 


MAGNETISM   AND   ELECTRICITY 


323 


•  >« 
6 


NUMERALS. 
3 


8 


4 

•  ••  i 

9 


Period, 


'-•          m 


PUNCTUATION. 
Comma,  Semi-colon. 


Quotation. 

•  -  J*—  -    mmm  • 


^aj^ejntjiesis^         Interrogation.  Colon.  Paragraph, 

298.  The  Relay  Used  in  Telegraphy.— When  the  distance 
to  be  telegraphed  is  very  great,  the  resistance  of  the  tele- 
graph wire  to  the  passage  of  the  current  of  electricity 
may  be  so  great  that  the  electromagnet  of  the  sounder 


does  not  become  magnetic  enough  to  pull  down  the  arma- 
ture in  opposition  to  the  action  of  the  spring  which 
tends  to  hold  the  armature  up.  In  that  case  another  in- 
strument is  used  called  the  relay  (Fig.  133).  This  instru- 
ment is  essentially  like  the  sounder.  The  electromagnet 


324  ELEMENTARY   PHYSICS 

of  the  relay  is,  however,  fastened  in  a  horizontal  position. 
The  armature,  A,  is  a  thin  vertical  piece  of  soft  iron  sup- 
ported by  a  horizontal  brass  bar  in  such  a  manner  as  to 
be  readily  movable.  Ordinarily  a  very  light  spring  draws 
the  armature  a  slight  distance  away  from  the  electromag- 
net. However,  as  soon  as  a  current  of  electricity  passes 
through  the  electromagnet,  the  armature  is  drawn  over 
towards  the  magnet,  N  S. 

Through  the  upper  end  of  the  armature  of  the  relay 
passes  a  small  platinum  peg,  B.  This  peg  during  its  for- 
ward and  backward  motions  strikes  against  the  tips  of 
two  horizontal  screws,  C  D,  passing  through  the  upper 
end  of  a  brass  post  bent  at  the  top  into  the  form  of  an 
arch,  P.  The  clicks  against  these  screws,  although  faint, 
may  be  used  for  purposes  of  telegraphy  just  as  were  the 
much  louder  sounds  produced  by  the  horizontal  brass 
tube  in  the  case  of  the  sounder.  In  order  to  make  the 
electromagnet  of  the  relay  as  strong  as  possible,  not- 
withstanding the  weak  currents  passing  through  the 
wires,  the  coils  of  the  electromagnet  are  made  of  many 
turns  of  fine  wire.  By  using  fine  wire  it  is  possible  to 
get  many  more  turns  of  wire  on  the  same  electromagnet 
and  the  magnetic  strength  of  the  electromagnet  is  much 
increased. 

In  actual  practice  the  clicks  of  the  armature  in  case  of 
the  relay  are  never  used  for  purposes  of  telegraphy.  But 
when  the  armature  of  the  relay  touches  the  screw,  D,  on 
the  side  nearest  the  electromagnet  it  completes  the  path 
for  a  second  current  of  electricity  which  passes  through 
a  second  circuit  whose  total  length  is  usually  only  a  few 
feet.  In  this  second  circuit,  by  means  of  the  connections 
at  E  and  F,  are  placed  an  ordinary  sounder  and  also  an 
electric  cell,  so  that  whenever  a  current  of  electricity 


MAGNETISM   AND   ELECTRICITY  325 

passes  through  the  electromagnet  of  the  relay,  the  touch- 
ing of  the  platinum  peg  in  the  armature  against  the  metal 
tipped  screw  permits  a  current  to  pass  from  the  cell 
through  the  sounder.  When  no  current  passes  through 
the  electromagnet  of  the  relay,  the  armature,  A,  is  not 
attracted :  It  may  then  be  drawn  away  from  the  electro- 
magnet by  the  weak  spring.  This  stops  also  the  cur- 
rent in  second  or  local  circuit.  Even  if  the  platinum 
tip,  B,  strikes  against  the  screw,  C,  no  current  can  flow 
since  the  tip  of  this  screw  is  made  of  ebonite,  a  non-con- 


ductor (§  317)  of  electricity.  On  account  of  the  short 
length  of  the  second  circuit,  the  current  of  this  circuit  is 
much  stronger.  The  sound  produced  by  the  sounder  is 
much  louder  than  that  produced  by  the  relay,  and  the 
sounder  is  the  only  instrument  to  which  the  telegrapher 
pays  any  attention. 

299.  The  Telegraph  Key.— In  order  to  start  and  to  stop 
the  current  of  electricity  quickly  whenever  desired,  a 
telegraph  key  (Fig.  134)  is  used.  This  is  so  constructed 
that  when  the  key  is  pressed  down  there  is  no  break  in 
the  path  of  the  electricity.  But  when  the  finger  is  raised, 


326  ELEMENTAEY  PHYSICS 

the  key  is  thrown  up  by  a  spring,  S,  there  is  a  break  in 
the  circuit  at  B,  and  the  current  ceases  to  flow. 

300.  The  Earth  Forms  a  Part  of  the  Circuit  for  the  Elec- 
tric Current  Used  in  Telegraphy. — A  single  wire  may  con- 
nect all  the  stations  for  a  distance  of  several  hundred 
miles.     In  this  case,  the  ends  of  the  wires  at  the  extreme 
stations  are  connected  with  metal  plates  buried  so  deep 
in  the  ground  that  they  are  always  moist  (Fig.  135).     The 
electricity  then  passes  from  the  electric  cells  to  the  ex- 
treme end  of  the  wire  into  the  ground  and  then  returns 
through  the  moist  earth  by  way  of  the  plate  at  the  op- 
posite end  of  the  line.     That  part  of  the  circuit  which  is 
formed  by  the  earth  is  usually  known  as  the  ground  cir- 
cuit. 

301.  A  Message  Can  Be  Heard  at  all  Stations  on  the 
Same  Line. — Since  all  the  relays  at  all  of  the  stations  are 
connected  by  the  same  wire  (Fig.  135),  when  one  sounder 
is  heard  all  the  sounders  at  the  other  stations  on  the 
same  line  are  also  in  operation.     Any  message  heard  at 
one  station  may  therefore  also  be  heard  at  all  of  the 
others.      Unless    called  for  by  his  particular  number, 
the  telegrapher  need  pay  no  attention  to  the  message 
sent. 

302.  Arrangement  of  Electric  Cells  for  Telegraphic  Pur- 
poses.— When  it  is  desired  to  send  a  current  of  electricity 
through  a  long  wire  or  through  anything  which  offers  a 
great  resistance  to  the  passage  of  the  current,  the  cells 
are  arranged  in  series.     In  this  case  the  zinc  in  each  cell 
is  connected  with  the  copper  or  carbon  of  the  next  cell  in 
such  a  manner  that  the  current  of  electricity  must  pass 
from  cell  to  cell  before  it  can  get  out  into  the  main  wire 
(Fig.  135,  explanatory  drawings).     When  it  is  desired  to 
produce  a  strong  current  through  a  short  wire,  or  through 


MAGNETISM  AND   ELECTRICITY  327 


Ground  Plate 


Gro.und  Plate 


Zinc  plate 

Carbon  01  copper  plate 

|  I  ^sjji-g^e  dentate  cell 


Four  cells  arrangedjn56rie5 


|    I  |       |   l||  Also /our  cells  arrangetLiii  series 
Heetromagnet  of  Sounder  and  Relay 

ure  of  Sound'er  and  .Relay  with  Spring 


j  ^  Armat 


*^f    Key  whichjnay  he  closed  "by  side  lever 

FIG.  135., 


328 


ELEMENTARY   PHYSICS 


anything  offering-  little  resistance,  the  parallel  arrange- 
ment of  cells  is  used  (Fig.  128).  For  purposes  of  teleg- 
raphy a  number  of  gravity  cells  arranged  in  series  is 
commonly  employed. 

303.  The  Electric  Bell, — For  ringing  an  electric  bell  any 
kind  of  sal  ammoniac  cell  will  do.  Instead  of  a  key,  a  push 
button  is  used,  which  is  more  simple  in  construction  than 


FIG.  130. 

the  telegraph  key,  but  serves  for  exactly  the  same  pur- 
pose, to  open  and  to  close  the  circuit. 

In  the  electric  bell  (Fig.  136),  the  armature,  A,  is  held 
by  a  spring,  S,  a  short  distance  away  from  the  poles  of 
the  electromagnet.  The  spring  consists  of  a  flat  strip  of 
brass,  one  end  of  which  is  fastened  to  a  metal  post,  P,  and 
the  other  end  is  narrowed  and  so  bent  that  when  there  is 
no  current  in  the  electromagnet,  the  narrowed  end  of  the 
spring,  B,  presses  against  the  tip  of  the  screw,  C.  The 
current  after  passing  through  the  electromagnet  enters 


MAGNETISM   AND   ELECTRICITY  329 

the  post  supporting-  the  spring1,  P,  to  which  the  armature 
is  fastened.  It  passes  along-  the  spring-,  S,  to  the  nar- 
rowed end,  B,  and  there  it  enters  the  screw,  C. 

As  soon  as  the  current  passes  through  the  electromag- 
net, the  electromagnet  becomes  magnetic,  attracts  the 
soft-iron  armature,  A,  notwithstanding  the  action  of  the 
spring,  and  in  this  manner  draws  the  narrowed  end  of  the 
spring  away  from  the  screw.  As  soon  as  the  tip  of  the 
spring  leaves  the  screw,  the  path  of  the  current  is  broken, 
the  flow  of  electricity  ceases,  the  electromagnet  is  no 
longer  magnetic,  the  armature  is  no  longer  attracted,  and 
the  spring  to  which  it  is  attached,  on  assuming  its  orig- 
inal position,  throws  the  armature  back.  This,  however, 
brings  the  tip  of  the  spring  against  the  screw,  the  current 
is  re-established,  and  the  armature  is  once  more  attracted. 
The  armature  continues  to  fly  back  and  forth  as  long  as 
the  hand  is  pressed  against  the  push  button.  A  tapper  is 
attached  to  the  end  of  the  armature  and  a  bell  is  placed 
on  the  same  side  as  the  electro-magnet.  Whenever  the 
armature  is  drawn  over,  the  tapper  strikes  against  the 
bell. 

304.  Galvanometers, — If  a  wire  is  bent  around  a  compass 
so  that  the  current  goes  northward  over  the  needle  and 
southward  under  the  needle,  both  the  current  above  the 
needle  and  the  current  below  the  needle  will  tend  to 
throw  the  north-seeking  end  of  the  needle  westward. 
This  may  be  easily  shown  by  applying  the  thumb  and 
finger  rule  for  both  parts  of  the  wire.  If  the  direction  of 
the  strain  in  the  ether  around  the  wire  above  and  below 
the  needle  be  represented  by  lines  of  force,  it  is  evi- 
dent that  the  lines  of  force  due  to  both  the  current  above 
and  below  the  needle  will  point  in  the  same  direction 
within  the  space  between  the  wires  where  the  needle  is 


330 


ELEMENTAEY   PHYSICS 


situated.  The  wire  bent  in  the  manner  directed  forms  a 
single  loop.  If  instead  of  a  single  loop  a  great  many  turns 
of  wire  are  taken  the  current  will  pass  frequently  north- 
ward over  the  needle  and  southward  under  the  same,  and 
the  north-seeking-  end  of  the  needle  will  be  thrown  farther 
toward  the  west. 

In  the  case  of  any  given  coil,  the  amount  of  deflection 
of  the  needle  toward  the  west  will  increase  with  the  in- 
crease in  strength  of  the  current.  Upon  this  principle 


137. 

instruments  are  constructed  for  the  especial  purpose  of 
determining  the  strength  of  currents.  They  are  called 
galvanometers. 

The  tangent  galvanometer  (Fig.  137)  usually  consists  of 
either  few  turns  of  thick  wire  or  of  many  turns  of  thin 
wire,  forming  a  coil  eight  inches  or  more  in  diameter. 
At  the  centre  of  this  coil  is  placed  a  short,  thick  compass 
needle.  To  this  needle,  transversely  to  its  length,  is 
attached  a  very  light  aluminum  pointer  which  indicates 
the  distance  through  which  the  needle  has  turned  tow- 


MAGNETISM  AND   ELECTRICITY 


331 


ard  the  east  or  toward  the  west.  This  distance  is  read 
in  degrees  by  use  of  a  scale  attached  to  the  interior  of  the 
box  containing  the  needle.  The  coil  is  placed  in  a  north 
and  south  position  and  from  the  amount  of  deflection  of 
the  needle  toward  the  east  or  toward  the  west,  from  the 
strength  of  the  magnet,  from  the  size  of  the  wire  used  in 


-—Silk  fiber 


Mirro 


the  coil,  and  the  number  of  turns  of  the  wire,  the  strength 
of  the  current  may  be  determined. 

Some  galvanometers  are  made  of  a  great  many  turns  of 
wire.  The  magnetic  needle  is  made  very  light,  and  is 
suspended  from  a  single  thread  of  silk  so  that  even  a  very 
weak  current  will  serve  to  deflect  the  needle.  Such  in- 
struments may  be  called  sensitive  galvanometers.  When 
a  very  light  tiny  mirror  is  fastened  to  the  needle,  any 


332  ELEMENTARY   PHYSICS 

motion  of  the  needle  may  be  detected  by  throwing  light 
upon  the  mirror  so  that  it  will  be  reflected  upon  the  wall 
or  upon  a  screen.  Any  motion  of  the  mirror  causes  the 
spot  of  light  thrown  upon  the  screen  to  move.  An  instru- 
ment of  this  type  is  called  a  mirror  galvanometer  (Fig. 
138).  A  sensitive  mirror  galvanometer  will  be  found  ex- 
tremely useful  in  demonstrating  the  presence  of  the  weak 
currents  produced  during  many  of  the  following  experi- 
ments. 

305.  An  Electric  Current  Generated  in  a  Wiro  while  it  is 
Cutting  Lines  of  Magnetic  Force. — It  has  already  been 
shown  that  the  direction  of  the  strain  in  the  ether  may 
be  represented  by  lines  of  force  which  leave  the  north- 
seeking  end  of  the  magnet,  curve  around  through  the  air, 
and  enter  the  south-seeking  end.  In  the  case  of  a  per- 
manent horseshoe  magnet  many  of  the  lines  of  force  pass 
in  a  nearly  straight  direction  from  the  north-seeking  pole 
of  the  magnet  to  the  south-seeking  pole  (Fig.  121).  If 
the  magnet  is  so  held  that  the  north-seeking  pole  of  the 
horseshoe  magnet  is  directly  above  the  south-seeking 
pole,  the  direction  of  strain  may  be  represented  by  lines 
of  force  passing  directly  downward  within  the  space  be- 
tween the  poles.  If  then  the  magnet  is  turned  so  that 
the  poles  face  northward  (Fig.  139),  and  a  horizontal  wire 
is  moved  southward  within  the  space  between  the  poles, 
the  direction  of  motion  is  transverse  to  the  direction  of 
strain  in  the  ether.  Since  this  direction  of  strain  is  rep- 
resented by  lines  of  force,  it  may  also  be  said  that  the 
wire  cuts  the  lines  of  force. 

The  result  of  the  motion  of  the  wire  across  the  mag- 
netic field  is  a  change  in  the  direction  of  the  strain  in  the 
ether  in  the  immediate  vicinity  of  the  wire.  It  has  al- 
ready been  shown  that  a  piece  of  soft  iron  placed  in  the 


MAGNETISM  AND   ELECTRICITY 


333 


field  of  a  mag-net  alters  the  direction  of  the  strain.  The 
mere  placing1  of  a  piece  of  copper  or  other  metal  which  is 
not  magnetic  does  not  sensibly  alter  the  direction  of  the 
strain,  but  the  motion  of  a  copper  wire  across  the  magnetic 
field  in  some  manner  causes  a  change  in  the  direction  of 
this  strain.  The  strain  of  the  ether  directly  in  front  of  the 
moving  wire  appears  to  be  increased,  and  the  direction 
of  the  strain  at  the  side  and  the  rear  is  altered  in  such 
a  manner  as  to  curve  partly  or  entirely  around  the  wire. 
This  may  be  represented  diagrammatically  by  drawing 


.Direction  of  motion  of  wire 


the  lines  of  force  so  as  to  be  crowded  together  on  the 
side  in  front  of  the  moving  wire,  with  the  lines  of  force 
nearest  the  wire  entirely  encircling  it,  leaving  a  some- 
what smaller  number  of  lines  of  force  in  the  space  im- 
mediately in  the  rear. 

The  circular  strain  around  the  wire  is  the  combined 
result  produced  by  the  original  strain  of  the  ether  from 
pole  to  pole,  the  increased  strain  in  front  of  the  moving 
wire,  and  the  decreased  strain  behind. 

It  has  been  demonstrated  that  when  a  current  passes 
through  a  wire  the  strain  may  be  represented  by  lines  of 


334  ELEMENTARY   PHYSICS 

force  which  pass  around  it  in  circles.  From  this  it  may 
be  expected  that  the  direct  opposite  is  also  true,  so  that 
when  by  any  means  the  direction  of  strain  in  any  mag- 
netic field  is  so  altered  that  it  may  be  represented  by  lines 
of  force  passing  around  a  wire,  at  least  in  the  immediate 
vicinity  of  the  wire  as  in  the  case  of  the  present  discus- 
sion, a  current  will  tend  to  pass  along  the  wire.  If  the 
wire  is  connected  with  a  sufficiently  sensitive  mirror  gal- 
vanometer, the  existence  of  a  current,  when  the  wire  is 
moved  between  the  poles  of  the  magnet,  may  be  easily 
proved. 

By  reversing  the  thumb  and  finger  rule,  the  direction 
of  the  current  in  the  wire  may  be  predicted.  The  direc- 
tion of  the  strain  in  the  ether  in  front  of  the  moving 
wire  always  remains  similar  to  the  original  direction 
of  the  strain  in  the  magnetic  field  between  the  poles, 
even  in  the  immediate  vicinity  of  the  wire  where  the 
lines  of  force  are  represented  as  encircling  the  wire. 
Therefore,  grasp  the  wire  with  the  right  hand  so  that 
the  finger  tips  touch  that  side  of  the  wire  toward  which 
the  wire  is  moving  and  at  the  same  time  point  toward 
the  south-seeking  pole  of  the  magnet.  The  extended 
thumb  indicates  the  direction  of  the  current  in  the 
wire. 

In  the  preceding  case  a  horizontal  wire  is  supposed  to 
be  moving  southward  between  the  northward-facing  poles 
of  a  horseshoe  magnet,  the  north-seeking  pole  being  above 
the  south-seeking  one.  The  application  of  the  thumb  and 
finger  rule  indicates  that  the  current  passes  eastward 
through  the  wire.  If  the  wire  is  moved  northward  while 
the  poles  remain  in  the  same  position,  the  current  passes 
westward  through  the  Avire. 

It  is  noted  in  a  preceding  paragraph  that  a  magnetic 


MAGNETISM  AND   ELECTRICITY 


335 


field  encircles  a  wire  only  while  a  current  is  passing 
through  the  wire.  The  reverse  is  also  true.  In  order  that 
the  magnetic  field  may  produce  a  current  in  a  wire  its 
strain  must  be  of  such  a  character  as  to  encircle  the  wire* 
so  that  it  may  be  represented  by  lines  of  force  passing 
around  the  wire.  This  is  true  only  while  the  wire  is 
kept  in  motion  transversely  to  the  original  strain  in  the 
magnetic  field  of  the  magnet.  Therefore  the  current 
in  the  wire  ceases  as  soon  as  the  motion  of  the  wire 
ceases. 

306.  The  Dynamo.— If  a  single  loop  of  wire  placed  within 
the  space  between  the  poles  is  so  rotated  that  the  upper 
part  of  the  wire  loop  moves  southward  at  the  same  time 
that  the  lower  part  of  the  loop  moves  northward  (Fig. 
146,  A  and  B),  a  current  will  pass  eastward  in  the  upper 
part  of  the  loop  at  the  same  time  that  another  current 
passes  westward  in  the  lower  part. 
A  brief  examination  will  at  once  show 
that  these  currents  in  no  manner  in- 
terfere with  each  other,  but  that  one 
current  follows  immediately  behind 
the  other  around  the  loop. 

If  instead  of  a  permanent  horseshoe 
magnet  a  very  power- 
ful electromagnet  (Figs. 
140,  141)  is  used,  if  in- 
stead of  a  single  loop  of 
wire  a  coil  consisting  of 
several  hundred  loops 
is  employed  (Figs.  141,  FlQ  140 

145),  and  if  the  coil  is 

rotated  more  rapidly  so  as  to  complete  a  greater  number 
of  revolutions  per  second,  the  quantity  of  current  pro- 


336  ELEMENTARY   PHYSICS 

duced  by  a  single  revolution  of  the  coil  is  very  much  in- 
creased. 

We  have  already  learned  that  the  strain  of  ether  in  a 
magnetic  field  is  always  very  much  increased  by  the  pres- 
ence of  soft  iron.  If,  therefore,  the  space  between  the 
poles  within  which  the  coil  rotates  is  filled  as  much  as 
possible  with  soft  iron,  the  strength  also  of  this  field  is 
very  much  increased,  and  the  rotation  of  the  coil  within 
this  stronger  field  produces  a  stronger  current  of  elec- 
tricity. In  diagrammatic  drawings  this  is  represented  by 
drawing  a  greater  number  of  lines  of  force  through  the 
field  when  the  space  between  the  poles  is  largely  occu- 
pied by  soft  iron.  The  rotation  of  the  coil  then  causes 
the  wire  to  cut  a  greater  number  of  lines  of  force.  In 
practice,  the  field  within  which  the  coil  rotates  is  most 
effectively  filled  with  soft  iron  by  constructing  a  soft-iron 
core,  with  grooves  left  for  the  wire  coils.  The  wire  form- 
ing the  coils  is  wrapped  lengthwise  in  the  grooves  around 
this  core.  Those  parts  of  the  inner  sides  of  the  poles  of 
the  electromagnet  which  lie  nearest  to  the  coils  are  given 
a  concave  curvature  so  as  to  leave  the  air  space  between 
the  coils  and  the  poles  of  the  magnet  as  small  as  pos- 
sible (Fig.  140,  N,  S  ;  Fig.  141,  P). 

The  iron  core  with  its  coils  is  called  the  armature  (Fig. 
145,  D).  The  electromagnet  in  whose  field  the  armature 
revolves  is  called  the  field  magnet  (Fig.  140).  When  pro- 
vision is  made  to  permit  the  current  to  pass  from  the 
coils  of  the  armature  into  other  wires  so  that  the  cur- 
rent may  be  utilized,  the  combination  is  called  a  dynamo 
(Fig.  141). 

307.  The  Current  Produced  in  a  Loop  of  Wire  Rotated  in 
the  Field  of  a  Horseshoe  Magnet  Fluctuates. — If  attention 
be  again  confined  to  a  single  loop  of  wire  revolving  iu 


MAGNETISM   AND   ELECTRICITY  337 

the  field  of  a  permanent  horseshoe  magnet  (Fig.  139),  it  is 
seen  that  twice  during-  every  revolution,  while  the  loop  is 
horizontal,  the  wires  move  practically  parallel  to  the  direc- 
tion of  strain  in  the  field.  Under  these  circumstances,  the 
ether  is  not  strained  more  on  one  side  of  the  wire  than  on 


FIG.  141. 


the  other.  In  the  diagrammatic  representation  of  the 
field  of  the  magnet  it  will  be  seen  that  during-  this  part  of 
the  path  of  the  wire  of  the  loop  the  lines  of  force  are  not 
crowded  more  on  one  side  of  the  wire  than  on  the  other. 
Therefore,  during  this  part  of  its  revolution  no  circular 
field  is  produced  around  the  wire  and  the  wire  carries  no 


338 


ELEMENTARY   PHYSICS 


FIG.  142. 


current.     The  maximum  current,  on  the  contrary,  is  pro- 
duced when  the  wires  of  the  loop  are  moving  directly 

across  the  direction  of  strain 
of  the  magnetic  field.  This 
takes  place  while  the  rota- 
ting loop  occupies  a  practi- 
cally vertical  position.  Twice 
during  every  revolution,  the 
current  increases  from  zero 
to  the  maximum  current  and 
then  falls  again  to  zero.  This 

is  represented  diagrammatically  in  the  figure  (Fig.  142) 
by  varying  the  thickness  of  the  circular  path  followed  by 
each  wire  of  the  rotat- 
ing loop,  from  nothing, 
where   the    wire    moves 
parallel  to  the  lines  of 
force,  to  the  maximum 
thickness  where  it  cuts 
most  directly  across  the 
lines  of  force  and  thus 
produces  the  strongest  current.     The  irregularity  of  this 
current  may  be  largely  decreased  by  employing  two  loops 

at  right  angles  with  one  an- 
other (Fig.  143),  or  three  loops 
at  angles  of  sixty  degrees.  In 
figure  144  there  are  four  coils 
connected  with  each  other  in 
such  a  manner  that  the  current 
can  pass  from  coil  to  coil,  D, 
and  yet  escape  from  the  dy- 
namo at  the  proper  point  (B).  By  this  means  there  is  al- 
ways one  part  of  one  coil  very  near  that  part  of  its  course 


FIG.  143. 


FIG.  144. 


MAGNETISM   AND   ELECTRICITY 


339 


in  which  it  produces  its  maximum  current.  This  prevents 
the  current  from  decreasing-  very  much  in  strength  at  any 
time,  and,  therefore,  makes  it  more  steady.  Some  dynamos 


FIG.  145. 

have  armatures  constructed  upon  this  principle ;  they 
consist  of  a  considerable  number  of  coils  of  wire  placed 
at  different  angles  (Fig.  145,  D). 

308.  The  Current  in  the  Coil  Changes  in  Direction  Twice 
during  every  Revolution. — If  one-half  of  the  wire  loop  is 
painted  red  and  the  other  half  green,  it  becomes  evident 


Conducting 
ring- 


Brush 


FIG.  146.    A 


FIG.  146.     B 


that  the  red  half  of  the  wire  carries  an  eastward  going 
current  while  it  passes  from  the  extreme  north  position 
southward  (Fig.  146,  A).  However,  during  the  second 


340 


ELEMENTARY   PHYSICS 


half  of  its  revolution,  while  returning-  from  the  south  to 
the  north  side,  this  half  of  the  wire  loop  is  traversed  by 
a  westward-flowing1  current.  In  the  meantime,  the  cur- 
rent through  the  green  wire  has  changed  from  a  west- 
ward current  to  an  eastward  current  (Fig.  146,  B).  The 
result  is  that  the  current  in  the  coil  changes  in  direction 
twice  in  the  course  of  every  revolution. 

309.  Alternating  Currents. — A  current  continually  vary- 
ing in  direction  is  said  to  be  alternating.  All  currents 
produced  by  coils  rotating  in  the  fields  of  magnets  are 

alternating,  at  least  while 
the  currents  are  still  trav- 
ersing the  coil.  If  the  ends 
of  the  wires  of  a  rotating 
coil  were  fastened  directly 
to  wires  leading  to  any  in- 
strument, it  is  evident  that 
the  wires  would  soon  be- 
come twisted  and  broken. 
This  is  prevented  in  the 
following  manner. 

One  end  of  the  axle  carry- 
ing the  armature  with  its  coil  carries  also  two  brass  rings. 
One  of  the  ends  of  the  coil  is  fastened  to  one  ring  and  the 
other  end  of  the  coil  to  the  other  ring  (Fig-.  146).  These 
rings  must  be  separated  by  some  insulating  material  so 
that  the  current  cannot  pass  directly  from  one  ring  to 
the  other.  As  the  armature  rotates  and  the  current 
changes  in  direction,  the  current  tends  to  leave  the 
dynamo  first  by  one  ring  and  then  by  the  other.  If  two 
thin  strips  of  metal  press  against  these  rings,  the  cur- 
rent will  pass  from  the  rings,  first  into  one  and  then  into 
the  other  of  these  strips.  To  these  strips  of  metal,  per- 


FlG.  147. 


MAGNETISM  AND   ELECTRICITY  341 

manent  wires  leading  to  various  electrical  instruments 
may  be  fastened.  In  that  case  the  current  will  enter  al- 
ternately first  one  and  then  the  other  wire  brush,  pro- 
ducing- an  alternating  current  in  the  wire  connected  to  the 
brush.  The  two  rings  may  be  called  the  conducting  rings, 
and  the  thin  strips  of  metal,  the  brushes.  The  thin  strips 
of  metal  are  often  replaced  by  thick  pieces  of  carbon 
(Fig.  147^)  B^  and  formerly  were  constructed  of  actual 
wire  brushes.  In  figure  141,  the  carbon  brushes  are  en- 
closed in  brass  holders,  B,  and  only  the  lower  ends  of  the 
carbons  can  be  seen,  where  they  are  in  contact  with  the 
commutator,  C.  A  dynamo  with  conducting  rings  can 
produce  only  an  alternating  current. 

310.  An  Alternating  Current  Changed  into  a  Direct  Cur- 
rent by  Means  of  a  Commutator — If  instead  of  two  rings,  a 
single  ring  is  used,  and  if  this  ring  is  cut  transversely  into 
two  equal  halves,  the  ends  of  the  coil  connected  to  the 
half  rings  will  send  a  current  first  into  one  and  then  into 
the  other  half  ring.  The  two  half  rings  are  then  known 
as  a  split-ring  commutator  (Fig.  148).  In  this  case  the 
brushes  are  placed  on  opposite  sides  of  the  commutator 
at  such  an  angle  that  they  come  in  contact  with  both 
halves  of  the  commutator  only  at  the  moment  at  which 
the  coil  is  producing  practically  no  current.  In  other 
words,  while  the  wires  of  the  coil  are  moving  parallel  to 
the  lines  of  force.  If  the  coil  be  so  rotated  that  the 
upper  part  of  the  coil  always  moves  toward  the  south, 
then  that  half  of  the  coil  which  is  moving  southward  will 
always  produce  an  eastward- moving  current.  By  placing 
the  brushes  at  the  proper  angle,  it  may  be  so  arranged 
that  the  eastward-moving-  current,  no  matter  by  which 
half  of  the  loop  it  is  produced  or  by  which  half  of  the 
commutator  it  leaves,  will  always  leave  by  the  same  brush, 


342 


ELEMENTARY  PHYSICS 


while  the  returning-  current  will  always  re-enter  by  the 
other  brush.  In  other  words,  a  split-ring-  commutator 

may  be  used  to  secure 
currents  passing1  in  one 
direction  only. 

By  the  use  of  a  suf- 
ficient number  of  coils 
placed  at  various  an- 
gles with  each  other 
(Fig.  145,  D),  in  con- 
nection with  an  in- 
creased number  of 
segments  in  the  com- 
mutator  (Figs.  141, 145, 
147,  C),  a  current  may 

be  secured  which  is  not  only  constant  in  direction,  but 
also  fairly  constant  in  strength.  Dynamos  constructed  on 
this  principle  are  called  direct  current  dynamos  (Fig.  141). 

311.  The  Motor.— If 
a  strong  current  of 
electricity  be  sent  by 
way  of  the  brushes  in- 
to the  single  coil  of 
wire  found  in  a  simple 
dynamo  while  the  coil 
is  in  a  horizontal  posi- 
tion, in  such  a  manner 
that  the  upper  face  of 
the  coil  becomes 
north-seeking,  the  pja  148  B 

north-seeking  pole  of 

the  magnet  being  directly  above,— the  north-seeking  mag- 
netism of  the  coil  will  oppose  the  north-seeking  magnetism 


MAGNETISM   AND   ELECTRICITY 


343 


of  the  magnet.  The  upper  face  of  the  coil  and  the  north- 
seeking-  pole  of  the  magnet,  directly  above,  will  mutually 
repel  each  other  (Fig.  149),  and,  if  the  opposing  magnetic 
effects  are  sufficiently  strong,  the  coil  will  tend  to  move  so 
that  the  north-seeking  face  of  the  coil  will  turn  downward. 

If  a  half -ring  commutator  is  used,  the  current  may  be 
switched  into  the  coil  in  such  a  manner  that  at  the  mo- 
ment the  north-seeking  face  of  the  coil  is  directly  oppo- 
site the  south- seeking  face  of  the  magnet,  the  direction  of 
the  current  throughout  the  entire  coil  is  changed.  What 
was  the  north  -  seeking  — ^ 
face  of  the  coil  now  be- 
comes the  south-seeking 
face.  The  same  condi- 
tions are  therefore  re- 
produced as  at  the  be- 
ginning. The  coil  will 
therefore  rotate  once 
more  and  in  the  same 
direction.  In  other 
words,  when  a  divided- 
ring  commutator  is  used, 
the  coil  continues  to  re- 
volve as  long  as  it  is  supplied  with  a  sufficiently  strong 
current  of  electricity.  The  direct-current  dynamo  has 
become  the  direct-current  motor. 

A  motor  is,  therefore,  the  exact  reverse  of  a  dynamo. 
Power  is  necessary  to  revolve  the  coil  of  a  dynamo  and 
electricity  results.  Electricity  is  necessary  to  set  in 
operation  the  coils  of  a  motor  and  motion  results. 

312.  Induction. — Any  wire  carrying  a  current  is  sur- 
rounded by  a  field  within  which  magnetic  effects  are  per- 
ceptible, These  magnetic  effects  are  strongest  near  the 


FIG.  149. 


344  ELEMENTARY   PHYSICS 

wire  and  become  gradually  imperceptible  as  the  distance 
from  the  wire  becomes  greater.  Any  increase  in  strength 
of  the  current  passing  through  the  wire  is  accompanied 
both  by  an  increase  of  strain  in  the  ether  in  every  part  of 
the  magnetic  field  which  already  surrounds  the  wire  and 
by  an  extension  of  the  magnetic  field  to  a  greater  dis- 
tance from  the  wire.  Conversely,  any  decrease  in  the 
strength  of  the  current  results  in  a  decrease  of  both  the 
strength  and  the  extension  of  the  magnetic  field. 

If  around  one  coil  of  wire  a  second  coil  is  wrapped  in 
such  a  manner  that  the  wires  of  the  two  coils  at  no  point 
come  in  contact  with  each  other  (Fig.  151),  it  is  evident  that 
no  current  of  electricity  can  pass  directly  from  the  first  coil 
into  the  second.  However,  the  magnetic  field  around  the 
inner  coil  may  extend  to,  and  beyond  the  outer  coil.  Any 
increase  in  the  strength  of  the  current  passing  through 
the  inner  coil  must  result  in  an  increase  in  the  strain  of 
the  ether  around  the  wire  in  each  part  of  this  coil.  Tkis 
increased  strain  is  in  part  transmitted  outward,  affecting 
the  magnetic  field  even  beyond  the  outer  coil.  This 
spreading  of  the  increased  strain  seems  to  be  slightly 
checked  on  reaching  the  wire  forming  the  outer  coil.  In 
consequence,  the  direction  of  strain  of  the  ether  in  im- 
mediate contact  with  the  wire  forming  the  outer  coil  is 
altered  to  a  circular  strain  and  this  results  in  a  current 
through  the  wire. 

The  direction  of  the  current  in  the  wire  of  the  outer 
coil  due  to  the  increase  of  current  in  the  inner  coil  may 
be  determined  mechanically  by  the  use  of  the  line  of  force 
idea.  Any  increase  in  the  strength  of  the  current  passing 
through  a  wire  may  be  represented  by  an  increase  in  the 
number  of  lines  of  force  in  every  part  of  the  field  sur- 
rounding the  wire  and  also  by  the  spreading  of  these 


MAGNETISM   AND   ELECTRICITY 


345 


lines  of  force  so  as  to  occupy  a  larger  field.  Conversely, 
any  decrease  in  the  strength  of  the  current  may  be 
imagined  to  result  in  a  shrinking  of  the  lines  of  force  ac- 
companied by  a  diminution  of  their  number  and  also  by 
a  decrease  in  the  size  of  the  field. 


Primary  current 

increasing  in 

strength 


FIG.  150. 

If  in  the  vicinity  of  one  wire  carrying  a  current  there  is 
a  second  wire  (Fig.  150),  any  increase  in  the  strength  of 
the  current  passing  through  the  first  wire  must  result  in 
an  increase  in  the  number  of  lines  of  force  passing  around 
this  wire.  In  consequence,  the  lines  of  force  spread  to  a 
greater  distance  from  the  first  wire  and  strike  against  the 


346 


ELEMENTARY   PHYSICS 


second  wire,  which  so  far  has  not  contained  any  current. 
But  as  soon  as  the  lines  of  force  of  the  first  wire  strike 
against  the  second  wire,  they  begin  to  wrap  around  it 
and  this  results  in  the  production  of  a  current  in  the  sec- 
ond wire,  opposite  in  direction  to  the  current  present  in 
the  first  wire. 

By  studying  on  which  side  of  the  wires  of  the  outer 
coil  the  lines  of  force  spreading  from  the  inner  coil  are 
momentarily  held  in  check,  it  is  possible  to  determine  in 
what  direction  the  lines  of  force  will  pass  around  the 
wires  of  the  outer  coil,  and  what  must  be  the  direction 


n 


Secondary  Current 

J\  (\  J\  AJ\fU\AJ\OfU\fU 


Induction  due  to  increase 
in  strength  of  current 


A  in  strength  ol  ci 
in  primary  coil 


FIG.  151. 

taken  by  the  current  in  the  outer  coil.  The  current  in- 
duced in  the  secondary  coil  is  found  always  to  be  opposite 
in  direction  to  that  present  in  the  primary  coil  (Fig.  151). 
In  one  particular,  the  process  is  the  exact  reverse  of 
that  which  is  followed  when  a  current  is  produced  in  a 
wire  by  moving  it  so  as  to  cut  directly  across  the  lines  of 
force  between  the  poles  of  a  magnet.  In  one  case,  the 
spreading  of  lines  of  force  causes  the  lines  of  force  to 
move  toward  and  strike  against  the  wire.  In  the  other 
case  the  wire  is  moved  toward  and  strikes  against  the 
lines  of  force.  In  both  cases  the  lines  of  force  wrap 
around  the  wire  and  a  Current  results  in  the  wire. 


MAGNETISM   AND   ELECTRICITY  347 

When  the  current  in  the  inner  coil  decreases,  the  lines 
of  force  again  contract.  The  result  is  a  current  in  the 
opposite  direction  from  that  first  produced  in  the  outer 
coil.  Increase  and  decrease  of  current  in  the  inner  coil 
therefore  results  in  an  alternation  in  the  direction  of  the 
current  in  the  outer  coil. 

The  two  coils  of  wire  used  in  the  manner  here  described 
may  be  called  induction  coils.  Although  two  coils  are 
always  present,  the  combination  is  usually  referred  to  in 
the  singular  number,  as  an  induction  coil.  The  name 
Ruhmkorff  coil  is  frequently  employed  when,  at  one 
point  in  the  outer  coil,  there  is  a  break  or  spark  gap, 
across  which  the  secondary  current  must  leap  in  order  to 
complete  the  circuit.  Induction  coils  are  used  with  tele- 
phones. They  are  usually  placed  in  the  box  below  the 
transmitter.  Ruhmkorff  coils  are  used  for  wireless  teleg- 
raphy (§  322)  and  Eoentgen  rays  (§  323),  although  Holtz 
and  Wimshurst  induction  machines  may  also  be  employed. 

313.  The  Telephone.— In  the  case  of  telephones,  the  in- 
strument into  which  you  speak  is  called  the  transmitter, 
and  that  by  means  of  which  you  hear  is  called  the  re- 
ceiver. Two  wires  lead  from  the  telephone  instrument  to 
the  telephone  station  and  from  there  pairs  of  wires  lead 
off  to  other  telephones  in  the  same  town  or  district.  The 
connections  between  the  wires  from  your  telephone  and 
those  leading  to  any  other  house  are  made  at  the  central 
station. 

In  the  most  recent  systems  in  use  the  electric  cells 
(storage  cells)  are  all  located  at  the  central  station.  As 
soon  as  the  receiver  is  lifted  from  its  hook,  the  trans- 
mitter is  in  electrical  connection  with  the  central  office, 
and  there  the  connection  is  made  with  any  other  tele- 
phone. 


348 


ELEMENTAEY  PHYSICS 


314.  The  Transmitter.— The  transmitter  (Fig.  152)  con- 
sists of  a  wooden  box  with  an  opening  in  front,  behind 
which  is  a  thin  piece  of  iron,  called  a  diaphragm,  against 
which  you  talk.  The  sound  waves  produced  by  the  voice 
strike  against  this  diaphragm  and  push  the  central  part 
forward.  Between  each  sound  wave  the 
diaphragm  flies  back.  When  it  is  re- 
membered that  sounds  of  high  pitch  are 
caused  by  the  production  of  many  thou- 
sands of  successive  sound  waves  in  a  sin- 
gle second,  it  is  almost  in- 
conceivable that  even  the 
thinnest  metal  plate  should 
respond  to  all  of  these  waves. 
Nevertheless  this  is  actually 
the  case,  and  every  wave 
produced  by  the  voice,  or  by 
any  other  sound-producing 
instrument,  affects  to  some 
degree  the  motion  of  the  di- 
aphragm. A  screw  connects 
the  centre  of  the  diaphragm 
with  the  cover  of  a  round 
brass  box  which,  however, 
•looks  more  like  a  round  flat 
piece  of  brass  as  seen  from 
the  rear,  on  opening  the  door 
of  the  transmitter.  The  cover  of  this  box  consists  of  a 
thin  sheet  of  mica  to  the  centre  of  which  is  attached  a  thin 
brass  disk.  The  screw  already  mentioned  connects  the 
diaphragm  with  the  brass  disk.  As  the  diaphragm  moves 
backward  and  forward  the  brass  disk  also  moves  in  the 
same  manner.  To  the  inner  side  of  the  brass  disk  and  to 


FIG.  152. 


MAGNETISM   AND   ELECTKICITY  349 

the  base  of  the  box  directly  opposite  are  fastened  two 
round  and  highly  polished  carbon  disks.  Between  these 
disks  the  box  is  filled  with  minute  carbon  grains. 

The  current  comes  from  the  battery  at  the  central  tele- 
phone station  to  the  receiver  and  passes  by  a  thin  con- 
necting wire  to  the  brass  disk  on  the  cover  of  the  box. 
Thence  it  passes  through  the  carbon  disk  attached  to  the 
brass  disk,  through  the  carbon  grains  to  the  carbon  disk 
at  the  base  of  the  box,  and  finally  through  the  brass  box 
itself  by  another  thin  wire  to  the  wire  returning  to  the 
station. 

Whenever  a  sound  wave  throws  the  diaphragm  for- 
ward, the  cover  of  the  box  presses  the  carbon  grains  to- 
gether so  effectively  as  to  cause  them  to  transmit  elec- 
tricity better  than  when  the  diaphragm  moves  back  and 
the  carbon  grains  are  slightly  less  crowded.  Every  mo- 
tion of  the  diaphragm  therefore  causes  a  corresponding 
change  in  the  strength  of  the  current  passing  through  the 
transmitter,  and  therefore  through  the  wires  leading  to 
the  central  office  and  thence  to  the  other  telephone.  It  is 
difficult  to  conceive  that  the  slight  variations  in  compres- 
sion of  the  carbon  grains  should  cause  such  distinct  varia- 
tions in  the  strength  of  the  current  that  the  hundreds  or 
thousands  of  the  sound  waves  produced  by  the  voice  in 
the  coiirse  of  a  single  second,  result  in  a  corresponding 
number  of  variations  in  the  current  of  electricity  pass- 
ing through  the  transmitter.  This,  however,  is  the  case. 

315.  The  Receiver. — The  receiver  (Fig.  153)  consists  of 
a  long,  round  bar  magnet  with  the  north-seeking  end 
almost  touching  the  centre  of  the  metal  diaphragm.  The 
cover  of  the  receiver  may  be  easily  unscrewed  and  the  soft 
iron  diaphragm  removed.  There  is  then  seen  a  small 
spool  surrounding  the  north-seeking  end  of  a  permanent 


350 


ELEMENTAEY  PHYSICS 


bar  magnet  (Fig.  154).  The  spool  is  wrapped  with  many 
turns  of  very  thin  wire  covered  with  silk.  The  ends  of 
this  wire  pass  lengthwise  through  the  receiver,  from  the 
spool,  along  the  magnet,  to  the  inner  ends  of  the  binding 
posts  at  the  rear  of  the  instrument. 

When  a  current  is  sent  through  the  coil  of  wire  on  the 
spool  in  one  direction,  the  end  of  the  coil  facing  in  the 
same  direction  as  the  north-seeking  pole  of  the  permanent 
magnet  becomes  also  north-seeking  (Fig.  154,  A).  The 
magnetic  effect  produced  by  both  the  permanent  magnet 
and  by  the  coil  is  now  greater  than  that  produced  a  nio- 


FlG.  153. 

ment  before  by  the  magnet  alone.  The  thin  soft  iron 
diaphragm  is  therefore  attracted  more  strongly,  and  is 
drawn  slightly  nearer  to  the  end  of  the  magnet.  When, 
however,  the  current  is  sent  in  an  opposite  direction 
through  the  wire,  the  end  of  the  coil  facing  in  the  same 
direction  as  the  north-seeking  pole  of  the  magnet  be- 
comes south-seeking  (Fig.  154,  B).  The  south-seeking 
magnetism  of  the  coil  slightly  counteracts  the  north - 
seeking  magnetism  of  the  bar  magnet.  The  magnetic 
effect  produced  by  both  magnet  and  coil  is  then  less  than 
the  effect  usually  produced  by  the  magnet  alone.  The 


MAGNETISM   AND   ELECTKICITY 


351 


diaphragm  is  attracted  even  less  than  when  no  current 
in  any  direction  is  passing-  through  the  coil.  In  conse- 
quence the  diaphragm  springs  back  to  a  position  slightly 
more  distant  from  the  end  of  the  permanent  magnet  than 
when  no  current  is  passing.  Any  change  in  the  direc- 
tion of  the  current  through  the  coil  results  in  a  variation 
in  the  degree  of  attraction  of  the  diaphragm,  and  this 
causes  the  diaphragm  to  move  backward  and  forward. 

316.  The  Use  of  Induction  Coils  in  Telephone  Circuits.— 
It   has  been   shown   (§  312)  that   any  variation  in  the 


FIG.  154. 


strength  of  a  current  of  electricity  passing  through  a  coil 
of  wire  surrounded  by  another  coil  must  result  in  a  cur- 
rent passing  through  the  outer  coil.  Any  increase  in  the 
strength  of  the  current  in  the  inner  coil  produces  a  cur- 
rent in  the  opposite  direction  in  the  outer  coil,  while  any 
decrease  in  the  current  in  the  inner  coil  results  in  a  cur- 
rent in  the  same  direction  in  the  outer  coil.  It  has  also 
been  shown  (§  314)  that  the  variable  pressure  produced 
by  a  succession  of  sound  waves  striking  the  diaphragm  of 
a  telephone  transmitter  is  able  to  produce  a  variable  cur- 
rent through  the  transmitter. 


352  ELEMENTARY  PHYSICS 

If,  therefore,  the  variable  current  passing  through  the 
transmitter,  during*  the  use  of  a  telephone,  be  sent 
through  the  inner  one  of  two  induction  coils  (Fig.  151), 
secondary  currents  of  electricity  will  be  produced  in  the 
coil  immediately  surrounding  the  first.  These  secondary 
currents  will  vary  in  strength  and  direction  with  the  vari- 
ations in  the  strength  and  direction  of  the  current  in  the 
first  coil.  If  the  inner  coil  consists  of  comparatively  few 
turns  and  the  outer  coil  is  formed  by  a  much  greater  num- 
ber of  turns  of  wire,  the  current  produced  in  the  outer 
coil  will  move  with  greater  violence  along  a  greater 
length  of  wire  than  could  be  passed  over  by  the  current 
in  the  inner  coil. 

By  connecting  the  two  ends  of  the  wire  in  the  outer 
coil  with  the  two  binding  posts  at  the  rear  of  the  receiver, 
the  variations  in  the  strength  and  direction  of  the  current 
in  the  outer  coil  may  be  used  to  produce  currents  vary- 
ing in  strength  and  direction  in  the  coil  wrapped  around 
the  end  of  the  permanent  magnet  in  the  receiver.  This 
will  result  in  forward  and  backward  motions  in  the  dia- 
phragm of  the  receiver.  By  properly  connecting  the 
ends  of  the  wire  of  the  outer  coil  with  those  of  the  re- 
ceiver, every  motion  of  the  diaphragm  in  the  transmitter 
will  be  imitated  by  the  diaphragm  in  the  receiver. 

This  would  be  true  if  the  wires  leading  from  the  trans- 
mitter were  connected  directly  with  the  wires  of  the  coil 
around  the  spool  in  the  receiver.  However,  when  induc- 
tion coils  are  employed,  which  possess  a  much  greater 
number  of  turns  of  wire  in  the  outer  coil  than  in  the  inner 
one,  the  force  with  which  the  current  produced  in  the 
outer  coil  tends  to  move  is  much  greater  than  the  force 
shown  by  the  current  in  the  inner  coil.  Hence  the  sec- 
ondary current  produced  in  the  outer  coil  speeds  on  from 
the  outer  coil  to  the  receiver  with  much  greater  force 


MAGNETISM   AND   ELECTRICITY  353 

than  the  current  which  comes  from  the  transmitter  to  the 
inner  coil  of  the  induction  coils.  The  secondary  current 
produced  in  the  outer  coil,  therefore,  can  overcome  a 
much  greater  resistance  (§  318)  and  pass  through  a  much 
greater  length  of  wire  than  that  traversing  the  inner  coil. 
The  use  of  the  induction  coils  is  therefore  to  secure  a 
current  which  will  tend  to  move  through  the  street  wires 
to  the  receiver  with  much  greater  force  than  the  current 
coming  directly  from  the  transmitter,  and  at  the  same 
time  to  secure  a  current  whose  variations  in  strength 
and  direction  are  a  precise  duplicate  of  those  coming 
from  the  transmitter. 

317.  Action  of  the  Telephone. — Each  sound  wave  caused 
by  the  voice  pushes  the  diaphragm  of  the  transmitter  for- 
ward, and  this  produces  a  better  contact  between  the  car- 
bon particles.  The  result  is  an  increased  flow  of  electricity 
through  the  first  coil.  During  this  increase  of  current,  a 
momentary  current  passes  in  the  opposite  direction  in 
the  second  coil,  and  by  proper  connections  with  the  spool 
in  the  receiver,  the  diaphragm  in  the  receiver  may  like- 
wise be  caused  to  move  forward.  During  the  interval 
between  two  sound  waves,  the  diaphragm  of  the  trans- 
mitter flies  back,  and  in  consequence  the  carbon  particles 
of  the  transmitter  are  in  looser  contact  with  one  another 
and  a  smaller  current  flows  on  to  the  first  coil.  The  de- 
crease of  current  in  the  first  coil  is  accompanied  by  a 
momentary  current  in  the  second  coil  having  the  same 
direction  as  that  in  the  first  coil.  The  direction  of  the 
current  in  the  spool  of  the  receiver,  is  in  consequence 
reversed.  The  diaphragm  of  the  receiver,  therefore, 
moves  in  the  opposite  direction.  In  this  simple  manner 
the  diaphragm  of  the  receiver  is  caused  to  imitate  all  the 
motions  of  the  diaphragm  of  the  transmitter. 

The  diaphragm  of  the  transmitter  is  set  in  motion  by 


354  ELEMENTARY   PHYSICS 

the  waves  of  the  voice.  The  motions  of  the  diaphragm 
of  the  receiver,  however,  set  the  air  in  motion,  and  these 
motions  reproduce  the  original  sound. 

318.  Conductors. — The  ability  of  different  substances  to 
conduct  electricity  varies  considerably.  Metals  are  the 
best  conductors,  but  different  metals  vary  considerably  in 
conductivity.  Silver  and  copper  are  the  best  conductors. 
Soft  iron  and  platinum  conduct  only  one-sixth  as  well. 
German  silver  conducts  about  one-twelfth  as  well.  As 
compared  with  this  carbon  is  a  poor  conductor.  The 
power  to  conduct  electricity  varies  not  only  with  the  sub- 
stance, but  also  with  the  area  of  cross  section  of  the  wire, 
fibre,  or  rod  which  conducts  the  electricity.  On  this  ac- 
count a  thick  rod  of  carbon  may  serve  to  conduct  elec- 
tricity much  better  than  a  thin  copper  wire. 

The  electricity  may  pass  also  more  readily  through  a 
short  piece  of  wire  than  through  a  longer  one.  Electric- 
ity may  pass  most  readily  through  a  short  thick  piece  of 
wire  constructed  of  the  best  conducting  material,  silver  or 
copper.  It  may  pass  far  less  readily  through  a  long 
thin  piece  of  an  only  moderately  good  conductor,  such  as 
German  silver.  Any  body  which  does  not  readily  permit 
the  flow  of  electricity  through  it,  is  said  to  offer  resist- 
ance to  the  flow  of  electricity.  It  is  often  desirable  not  to 
have  too  great  a  flow  of  electricity  through  certain  ma- 
chines when  they  are  first  started.  This  may  be  managed 
by  causing  the  current  of  electricity  at  first  to  flow 
through  a  considerable  length  of  some  poor  conductor  of 
electricity.  After  the  machine  has  once  started  in  opera- 
tion, the  current  is  caused  to  flow  through  a  much  shorter 
length  of  the  poor  conductor,  thus  causing  the  larger 
part  of  the  current  to  flow  through  the  machine.  Boxes 
with  coils  arranged  for  the  purpose  of  controlling  the 


MAGNETISM  AND   ELECTRICITY 


355 


amount  of  flow  of  electricity  are  often  called  resistance 
boxes.  The  resistance  boxes  at  the  front  end  of  electric 
street  cars  control  the  quantity  of  current  admitted  to  the 
motor  beneath  the  car,  and  are  hence  called  controllers, 
Resistance  coils  are  also  used  in  connection  with  stereop- 
ticons  for  the  purpose  of  regulating  the  flow  of  electricity 
through  the  lamp. 

319.  The  Incandescent  Lamp. — Anything  which  offers  re- 
sistance to  the  flow  of  electricity  becomes  heated  in  con- 
sequence. The  same  flow  of  electricity  through  the  same 


Brass  contact  with 
wall  oflsocket 


FIG.  155. 

length  and  thickness  of  copper,  platinum,  and  carbon 
will  heat  the  platinum  more  than  the  copper,  and  the  car- 
bon far  more  than  either  of  the  other  two.  This  may  be 
seen  in  the  case  of  an  ordinary  incandescent  electric  light 
(Fig.  155).  In  this  case  the  carbon  filament  within  the 
lamp  becomes  very  much  more  strongly  heated  than  the 
very  short  pieces  of  platinum  which  connect  the  ends  of 
the  carbon  filament  within  the  lamp  to  the  copper  wires 
at  its  base.  The  result  is  that  the  carbon  filament  be- 
comes white  hot,  while  the  copper  and  platinum  wires 
become  heated,  but  not  sufficiently  to  produce  light. 


356  ELEMENTARY   PHYSICS 

320.  The  Arc  Light — Fasten  two  pointed  pieces  of  car- 
bon to  the  wires  leading  from  a  powerful  voltaic  battery 
or  from  a  dynamo  in  such  a  manner  that,  when  the  car- 
bons are  allowed  to  touch,  the  circuit  is  closed.     Bring 
the  points  of  the  carbons  in  contact  for  a  few  seconds  and 
then  slowly  separate  them.   The  points  of  the  carbons  be- 
come heated  to  incandescence,  and  a  luminous  arc  is  seen 
to  extend  between  them. 

When  the  carbons  are  permitted  to  touch,  a  current  passes 
through  them.  As  they  are  separated,  the  current  leaps 
across  the  small  gap  and  volatilizes 
some  of  the  carbon.  This  carbon 
vapor,  being  a  partial  conductor, 
permits  the  current  to  flow  across 
the  gap,  provided  the  gap  is  not 
too  wide.  The  ends  of  the  carbons 
are  more  brilliant  than  the  vapor 
between  them.  The  tip  of  the  car- 
bon connected  with  the  wire  from 
the  positive  pole  of  a  battery  or 
FIG.  156.  dynamo  is  slowly  consumed  so  that 

it    possesses   a   cup-shaped   cavity 

called  the  crater  (Fig.  156).     It  is  this  crater  which  is  the 
hottest  and  most  luminous  part  of  the  light. 

Since  the  carbons,  slowly  waste  away,  it  is  necessary  to 
keep  moving  them  toward  each  other  in  order  that  the 
distance  between  them  may  remain  practically  uniform. 
In  most  arc  lights  in  use  this  is  accomplished  by  an  auto- 
matic arrangement  controlled  by  an  electromagnet. 

321.  The  Production  of  Electric  Waves  by  Induction  Coils. — 
In  one  of  the  instruments  utilizing  induction  coils,  called 
the  Ruhmkorff  coil  (Fig.  157),  the  current  sent  through 
the  inner  coil  is  automatically  broken  and  re-established 


MAGNETISM   AND   ELECTKICITY 


357 


many  times  in  the  course  of  each  second  by  a  specially 
devised  mechanism.  The  violent  fluctuations  in  the 
strength  of  the  current  passing  through  the  inner  coil, 
produced  in  this  manner,  result  in  alternating  currents 
in  a  second  coil  of  wire,  wrapped  around  the  first.  The 
number  of  turns  of  wire  in  the  outer  coil  is  much  greater 
than  that  used  for  the  inner  coil.  In  large  instruments 
the  outer  coil  is  made  of  very  thin  wire  and  many  thou- 
sands of  turns  of  wire  are  present.  Hence  the  force  with 


Induction  or  Ruhmkorff  Coil 


FIG.  157. 


which  electricity  is  urged  to  move  onward  is  so  much 
greater  than  that  present  in  the  inner  coil  that  the  elec- 
tricity can  jump  across  a  considerable  gap  through  the 
air  if  the  circuit  belonging  to  the  outer  coil  is  anywhere 
broken. 

Since  the  current  of  electricity  in  the  outer  coil  alter- 
nates rapidly  in  direction,  in  consequence  of  the  rapid 
automatic  breaking  of  the  circuit  belonging  to  the  inner 
coil,  the  passage  of  electricity  through  the  air  or  spark 
gap  also  alternates  in  direction.  The  result  of  these  al- 
ternating discharges  of  electricity  through  the  air  is  the 


358  ELEMENTARY   PHYSICS 

production  of  a  succession  of  sparks  accompanied  by  os- 
cillations or  waves  in  the  ether.  The  production  of  these 
waves  in  the  ether  appears  to  be  much  facilitated  by  at- 
taching- a  long  vertical  wire  to  some  part  of  the  circuit 
belonging  to  the  outer  coil. 

A  closer  examination  of  the  discharges  of  electricity 
through  the  air-gap  has  shown  that  the  passage  of  elec- 
tricity across  the  air-gap  is  not  accomplished  by  a  single 
discharge  on  each  change  in  direction  of  the  current,  but 
that  each  apparent  single  discharge  consists  in  reality  of 
a  very  rapid  succession  of  discharges  alternating  in  di- 
rection. The  setting  up  of  waves  in  the  ether  is  there- 
fore a  somewhat  complicated  process. 

322,  Wireless  Telegraphy — The  oscillations  or  waves  in 
the  ether  produced  by  a  large  Kuhmkorff  coil,  to  which  a 
vertical  wire  has  been  attached,  may  be  used  for  purposes 
of  wireless  telegraphy.  At  the  sending  station  an  ordi- 
nary telegraph  key  is  inserted  in  the  circuit  including 
the  inner  coil  of  the  Kuhmkorff  coil  and  the  electric  cells 
(Fig.  157).  By  this  means,  the  length  of  time  during 
which  electricity  flows  through  the  inner  coil  may  be 
controlled  as  in  ordinary  telegraph  instruments.  The 
production  of  currents  of  electricity  in  the  outer  coil 
takes  place  only  when  the  telegraph  key  is  depressed, 
and  ceases  the  moment  the  key  is  permitted  to  spring 
back.  The  production  of  waves  in  the  ether  is  due  to  an 
alternating  discharge  of  the  electric  currents  in  the  outer 
coil  across  the  spark-gap. 

At  the  receiving  station  are  two  small  but  complete  cir- 
cuits (Fig.  158).  In  the  first  circuit  are  several  gravity 
cells,  which  send  a  current  through  the  electromagnet  of 
a  relay  and  through  a  glass  tube  containing  nickel  filings 
held  in  place  loosely  by  metal  (silver)  disks.  This  tube 


MAGNETISM  AND   ELECTEICITY 


359 


is  called  the  coherer.  Loose  nickel  filings  under  ordinary 
circumstances  offer  a  considerable  resistance  to  the  flow 
of  electricity,  but  when  struck  by  electric  waves  or  oscil- 
lations they,  in  some  mysterious  manner,  become  good 
conductors  of  electricity.  Therefore,  a  good  current  of 


Vertical  wire  attached  to 
tin  lasin  near  the  ceiling. 


FIG.  158. 


electricity  passes  through  the  coherer  at  the  receiving 
station  as  long  as  the  key  at  the  sending  station  remains 
depressed,  and  during  this  time  the  electromagnet  of  the 
relay  attracts  its  armature.  As  soon  as  the  key  at  the 
sending  station  is  permitted  to  spring  back,  the  produc- 
tion of  electric  waves  or  oscillations  ceases,  the  nickel  fil- 


360  ELEMENTARY  PHYSICS 

ings  lose  their  conductivity,  and  the  armature  of  the  relay 
is  permitted  to  be  drawn  back. 

The  armature  of  the  relay  serves  as  a  key  to  a  second 
circuit  at  the  receiving-  station.  This  circuit  includes  a 
sounder,  as  in  ordinary  telegraphy.  It  also  includes  an 
electromagnet  whose  armature  is  supplied  with  a  tapper. 
An  electric  bell  may  be  fitted  up  for  this  purpose.  This 
tapper  continues  to  strike  against  the  coherer  as  long  as 
the  current  flows  through  the  circuit.  By  this  means 
the  nickel  filings  in  the  coherer  are  shaken  up  and  lose 
their  conductivity  much  more  completely  than  by  the 
mere  cessation  of  electric  waves  in  the  ether.  On  this 
account  the  electromagnet  with  its  tapper  is  called  a  de- 
coherer.  As  long  as  electric  waves  arrive  from  the  Ruhm- 
korff  coil,  the  nickel  filings  are  caused  to  cohere  the  in- 
stant they  are  decohered  by  the  tapper.  Practically  this 
results  in  uninterrupted  cohering.  But  as  soon  as  the 
waves  cease  to  arrive,  the  hand  having  been  taken  off  the 
key  at  the  sending  station,  the  last  stroke  of  the  tapper 
leaves  the  nickel  filings  too  loose  to  convey  a  good  cur- 
rent. Hence  all  electric  action  ceases  also  at  the  receiv- 
ing station. 

323.  Roentgen  Rays.— The  ends  of  the  wires  forming  the 
outer  coil  of  a  Ruhmkorff  coil  (Fig.  159)  may  be  inserted 
at  the  extremities  of  a  large  tube  and  the  points  of  entry 
closed  air  tight.  The  discharge  of  electricity  will  then 
take  place  through  the  air  enclosed  within  the  tube.  If 
an  opening  exists  in  one  side  of  the  tube  through  which 
the  air  may  be  pumped  out,  the  effect  of  decrease  of 
density  of  air  upon  the  character  of  the  discharge  of  elec- 
tricity through  the  air-gap  may  be  studied. 

Dry  air  of  ordinary  density  offers  a  considerable  resist- 
ance to  the  passage  of  electricity,  and  the  discharge  of  elec- 


MAGNETISM  AND   ELECTKICITY 


361 


tricity  through  the  air  is  by  a  bright,  clearly  defined  path. 
As  the  density  of  the  air  becomes  less  the  discharges  of 
electricity  through  the  air  become  more  frequent  and  the 
light  less  intense.  Moreover,  the  path  followed  by  the  elec- 
tricity while  passing  through  the  air  is  less  distinct,  and 
the  air  surrounding  it  begins  to  glow  with  a  bluish  or  pur- 
ple light.  As  the  density  of  the  air  becomes  less,  the  path 
taken  by  the  electricity  gradually  becomes  invisible,  while 


Fluoroscope 


^Primary  -wires  attached  to 
"dynamo  or  powerful  'battery. 


FIG.  159. 


the  purple  glow  of  light  extends  until  it  fills  the  entire  tube. 
After  about  fj-jj-  of  the  air  has  been  withdrawn  from  the 
tube,  the  glow  of  light  due  to  the  passage  of  electricity 
takes  the  form  of  a  succession  of  regions  of  stronger  light 
alternating  with  regions  of  comparative  darkness.  Finally 
when  nearly  all  the  air  has  been  removed  the  discharge  of 
electricity  through  the  tube  becomes  nearly  invisible. 

The  passage  of  the  current  through  the  tube  is  now  ac- 
companied by  the  production  of  waves  at  the  end  of  one 


362  ELEMENTAKY    PHYSICS 

of  the  wires  entering-  the  tube,  the  end  by  which  the  elec- 
tricity is  supposed  to  leave  the  tube,  the  cathode  end. 
These  waves  have  the  power  of  passing  through  many 
substances  ordinarily  called  opaque,  such  as  flesh,  leather, 
most  kinds  of  clothing,  paper,  wood,  ebonite,  and  many 
other  substances.  They  pass  with  much  less  readiness 
through  most  metals,  and  through  thick  glass.  Since 
these  rays  are  capable  of  affecting  the  chemicals  on  a 
photographic  plate,  it  is  possible  by  their  means  to  take 
photographs  of  coin  enclosed  in  leather  pocket-books,  or 
of  bullets  lodged  in  the  flesh,  or  of  broken  bones  in  the 
case  of  a  broken  arm  or  leg.  These  rays  have  also  the 
power  of  making  certain  chemicals,  such  as  calcium  tung- 
state,  glow  in  the  dark.  By  holding  the  hand  in  a  dark 
room  between  the  source  of  the  rays  and  a  sheet  of  card- 
board covered  with  calcium  tungstate,  the  bones  may  be 
seen  as  dark  shadows  within  a  slightly  darkened  area 
due  to  the  passage  of  the  rays  through  the  flesh  (Fig. 
160).  Tubes  especially  constructed  for  the  study  of  dis- 
charges of  electricity  through  nearly  perfect  vacua  are 
called  Crooke's  tubes.  The  sheet  of  cardboard  covered 
with  calcium  tungstate  is  known  as  a  fluorescent  screen. 
It  is  usually  placed  at  one  end  of  a  box  whose  opposite 
end  fits  closely  against  the  face.  This  leaves  the  interior 
of  the  box  dark,  and  makes  it  unnecessary  to  perform  the 
experiment  in  a  dark  room.  The  box  with  its  fluorescent 
screen  at  the  end  is  called  a  fluoroscope.  The  rays  here 
described  are  called  either  X-rays  or  Roentgen  rays. 

324.  Electricity  and  Magnetism  Are  Phenomena  of  the 
Ether. — It  will  be  noticed  from  the  preceding  paragraph 
that  electrical  phenomena  do  not  cease  when  the  air  has 
all  been  practically  removed  from  a  Crooke's  tube.  From 
this  it  may  be  concluded  that  electrical  phenomena,  al- 


MAGNETISM  AND   ELECTEICITY 


363 


though  profoundly  affected  by  the  presence  of  other  sub- 
stances, are  after  all  mainly  phenomena  of  the  ether.     The 


FIG.  160. 


364  ELEMENTARY  PHYSICS 

passage  of  electrical  oscillations  through  the  air  suggest 
the  same  conclusion,  since  the  passage  of  these  oscillations 
in  wireless  telegraphy  not  only  vastly  exceeds  in  speed 
the  most  rapid  transmission  of  vibrations  of  sound  through 
the  air,  but  equals  in  speed  the  transmission  of  light 
through  space.  Since  magnetic  phenomena  always  ac- 
company the  passage  of  electricity,  magnetism  must  also 
be  a  phenomenon  of  the  ether.  In  what  way  various  sub- 
stances produce  the  various  known  modifications  of  these 
phenomena  of  the  ether  is  at  present  unknown. 

Electricity  gives  rise  to  so  many  curious  phenomena 
and  permits  so  many  interesting  applications  to  useful 
ends  that  it  is  impossible  to  describe  more  than  a  small 
part  in  an  elementary  book.  The  choice  of  the  material 
to  be  selected  under  these  circumstances  must  be  de- 
termined therefore  chiefly  by  its  usefulness  as  an  intro- 
duction to  the  subject.  Since  this  is  a  matter  of  judg- 
ment, each  writer  or  teacher  is  liable  to  make  a  different 
selection.  The  pupil  will  find  an  interesting  continuation 
of  this  subject  in  Sylvanus  Thompson's  Elementary  Les- 
sons in  Electricity  and  Magnetism. 


CHAPTER  Yin 

MECHANICS 

325V  Gravitation — It  requires  considerable  exertion 
merely  to  hold  a  cannon-ball,  even  if  no  effort  is  made  to 
carry  it  for  any  distance.  The  ball  acts  as  if  something- 
were  forcing  it  downward — and  yet  nothing-  touches  the 
ball  except  the  hand  and  the  air.  It  cannot  be  the  air 
which  is  urging-  the  ball  downward.  The  air,  rather,  re- 
sists the  effort  of  the  ball ;  for  have  we  not  learned  that 
the  upward  pressure  of  the  air  on'  the  lower  surface  of  the 
ball  must  be  greater  than  the  downward  pressure  of  the 
air  on  the  top  of  the  ball  (§§  51,  52)  ?  And,  does  this  not 
leave  an  excess  of  upward  acting  force  to  counteract,  to  a 
slight  extent,  the  effort  of  the  ball  to  move  downward  ? 
And,  if  the  ball  is  allowed  to  drop,  does  not  the  friction 
of  the  air  through  which  the  ball  must  drop  retard  the 
motion  of  the  ball  slightly? 

No  cause  for  the  motion  can  be  detected.  There  is  no 
visible  connection  between  the  ball  and  the  earth.  As  far 
as  can  be  seen  there  is  nothing  either  pushing  or  pulling 
the  ball.  The  only  fact  of  which  we  are  absolutely  cer- 
tain is  that  the  ball  seems  urged  toward  the  earth  by  a 
certain  force  which  we  call  gravitation.  The  strength  of 
this  force  can  be  measured  by  fastening  the  ball  to  a 
spring  balance  by  means  of  a  string. 

326.  Directions  in  Which  Gravitation  Acts  on  the  Surface 
of  the  Earth. — The  direction  in  which  the  force  acts  is 

365 


366  ELEMENTARY   PHYSICS 

shown  by  the  direction  of  the  string-.  This  direction  is 
straight  downward.  No  matter  to  what  portion  of  the 
surface  of  the  earth  the  ball  be  taken,  the  string  from 
which  it  is  suspended  will  always  point  downward.  Does 
this  mean  that  the  ball  always  tries  to  move  in  the  same 
direction  ?  We  who  have  studied  geography  know  that 
it  means  anything  but  that ;  for  the  earth  is  round,  and 
down  at  one  point  of  the  surface  of  the  earth  is  an  en- 
tirely different  direction  from  down  at  any  other  point. 

In  fact  down  practically  means  toward  the  centre  of 
the  earth.  No  matter  upon  what  portion  of  the  earth  the 
ball  may  be,  it  will  try  to  move  toward  the  centre  of  the 
earth. 

Even  at  the  same  part  of  the  surface  of  the  earth,  down 
does  not  mean  in  the  same  direction  at  all  times  ;  for  the 
earth  turns  round  on  its  axis,  and  the  direction  of  a  line 
pointing  toward  the  centre  of  the  earth  changes  at  every 
instant.  A  man  at  the  equator  may  look  up  at  noon  and 
again  at  midnight.  He  has  looked  up  in  both  cases  and 
yet  he  has  looked  in  exactly  opposite  directions.  Why 
do  not  those  people  fall  off  the  earth  who  live  on  the  side 
opposite  us  ?  Because  something  forces  them  toward  the 
centre  of  the  earth  just  as  we  are  forced  toward  the  centre 
of  the  earth. 

327.  The  Force  of  Gravitation  Due  to  the  Earth  is  Less  at 
Points  More  Distant  from  the  Centre  of  the  Earth. — The  force 
with  which  a  body  is  urged  toward  the  earth  is  not  the 
same  in  all  localities.  It  is  less  at  the  tops  of  high  moun- 
tains than  at  the  level  of  the  sea.  The  greater  the  dis- 
tance from  the  centre  of  the  earth,  the  weaker  is  this  force. 
This  fact  can  be  determined  experimentally  with  wonder- 
ful exactness,  but  the  experiments  involve  a  very  exact 
knowledge  of  the  laws  of  the  pendulum  (§§  337-342). 


MECHANICS  367 

When  we  move  away  to  points  more  distant  from  the 
centre  of  the  earth,  the  force  with  which  a  body  is  urged 
toward  the  earth  decreases  much  more  rapidly  than  the 
distance  of  the  body  from  the  earth  increases.  In  fact,  it 
has  been  found  that  the  strength  of  the  force  diminishes  in 
proportion  as  the  square  of  the  distance  of  the  body  from  the 
centre  of  the  earth  increases. 

328.  The  Earth  Attracts  the  Atmosphere  and  the  Moon. — 
Notwithstanding-  this  rapid  diminution  of  force,  the  air 
from  a  distance  of  fully  100  miles  is  urged  toward  the 
earth,  so  that,  as  the  earth  moves  through  space,  it  carries 
the  air  with  it.     The  atmosphere  is  not  the  only  material 
which  follows  the  earth  on  its  journey  through   space. 
No  matter  in  what  direction  the  earth  may  be  travelling 
in  its  path  around  the  sun,  it  is  always  accompanied  by 
the  moon.     Although  the  moon  is  240,000  miles  away,  it  is 
continually  being  urged  toward  the  earth.     If  it  were  not 
for  the  fact  that  the  moon  has  a  motion  of  its  own  which 
tends  to  carry  it  away  from  the  earth,  the  moon  would 
not  only  keep  near  the  earth,  but  would  long  ago  have 
come  crashing  into  the  earth. 

We  do  not  know  whether  the  force  urging  the  moon 
and  other  objects  toward  the  earth  is  in  the  nature  of  a 
push  or  a  pull.  It  is  usually  assumed,  however,  that  it 
is  a  pull  and  the  earth  is  said  to  attract  the  moon  and 
other  objects.  v 

329.  The  Earth  is  Attracted  by  the  Sun.— The  earth  is 
not  the  only  body  which  attracts  other  objects.    It  moves 
around  the  sun  at  an  enormous  rate  of  speed.     If  some- 
thing were  not  urging  the  earth  toward  the  sun,  it  would 
fly  off  and  leave  the  sun  just  as  a  stone  fastened  to  a 
string  and  whirled  rapidly  around  through  the  air  will 
start  off  in  a  straight  line  the  instant  the  string  breaks. 


368  ELEMENTARY   PHYSICS 

The  sun  seems  to  attract  the  earth,  just  as  the  earth  at- 
tracts the  moon.  In  the  same  manner  the  sun  attracts 
the  planet  Jupiter,  and  Jupiter  attracts  five  moons. 

330.  Every  Object  Attracts  Every  Other  Object — Nor  is 
the  power  of  attraction  confined  to  the  heavenly  bodies. 
Every  object  attracts  every  other  object.     Yery  interest- 
ing- experiments  have  been  devised  in  order  to  prove  this. 
In  one  of  these  experiments,  it  may  be  shown  that  two 
thin  glass  globes  filled  with  mercury  attract  each  other 
in  a  distinctly  perceptible  manner. 

If  every  object  attracts  every  other  object,  not  only  does 
the  earth  attract  the  iron  ball,  but  the  ball  must  also 
attract  the  earth.  From  this  follows  another  and  more 
striking  conclusion,  that  not  only  does  the  ball  fall  toward 
the  earth,  but  the  earth  also  falls  toward  the  ball.  This 
is  actually  the  case.  But,  in  order  to  understand  why 
the  motion  of  the  earth  toward  the  ball  is  not  percepti- 
ble, it  is  necessary  to  understand  what  is  meant  by  quan- 
tity of  motion. 

331.  Velocity,  Mass,  and  Momentum. — A  body  moving 
from  one  position  to  another  occupies  time.     The  amount 
of  time  consumed  by  a  body  in  passing  over  a  given  dis- 
tance depends  upon  its  rate  of  motion,  or  how  fast  the 
body  is  moving.     This  rate  of  motion  of  a  body  is  called 
its    velocity.     In   determining  the   velocity  of  a  moving 
body  not  only  must  the  distance  passed  over  be  con- 
sidered, but  also  the  time  which  is  consumed.    A  body 
which  moves  a  distance  of  5  feet  in  one  second  is  said  to 
have  a  velocity  of  5  feet  per  second. 

The  quantity  of  motion  of  a  body  is  called  momen- 
tum. Common  experience  has  taught  us  that,  if  two 
bodies,  one  heavy  and  the  other  light,  are  moving  with 
equal  velocities,  it  is  much  more  difficult  to  stop  the  heavy 


MECHANICS  369 

body  than  it  is  to  stop  the  light  one.  In  this  case  the 
body  having  the  greater  amount  of  material,  or  mass, 
has  the  greater  quantity  of  motion.  If  one  body  is  mov- 
ing faster  than  another  body  of  equal  mass,  the  more 
rapidly  moving  one  is  the  harder  to  stop.  The  momen- 
tum of  a  moving  body  depends,  therefore,  upon  both  its 
mass  and  its  velocity. 

If  two  bodies  have  equal  velocities,  but  the  mass  of  one 
is  four  times  the  mass  of  the  other,  the  quantity  of  motion 
of  the  greater  is  four  times  that  of  the  lesser  body.  If 
one  of  two  bodies  of  equal  mass  is  moving  four  times  as 
fast  as  the  other,  its  quantity  of  motion  is  four  times  that 
of  the  more  slowly  moving  body.  A  body  which  has  both 
four  times  the  mass  and  four  times  the  velocity,  as  com- 
pared with  another  body,  has  4  X  4  or  16  times  the  quan- 
tity of  motion  possessed  by  the  other  body.  Therefore, 
the  quantity  of  motion,  or  momentum,  of  two  bodies  may 
be  compared  by  comparing  the  product  of  the  mass  and 
the  velocity  of  one  body  with  the  product  of  the  mass 
and  the  velocity  of  the  other  body.  If  a  ball  weighing 
4  pounds  moves  4  feet  in  one  second,  it  has  the  same 
momentum  as  a  body  weighing  1  pound  moving  16  feet 
in  1  second.  This  does  not  mean  that  both  balls  would 
do  the  same  amount  of  injury,  if  they  should  strike 
another  object,  but  that  their  momentum  is  the  same. 

332.  Both  Bodies  Which  Attract  Each  Other  Will  Fall 
toward  Each  Other  if  Free  to  Move. — If  the  earth  falls  tow- 
ard the  ball,  while  the  ball  is  falling  toward  the  earth, 
the  momentum  of  both  the  earth  and  the  ball  is  the 
same.  But,  in  this  case,  since  the  mass  of  the  earth 
is  enormously  greater  than  the  mass  of  the  ball,  the 
distance  moved  by  the  earth  in  one  minute  must  be 
entirely  imperceptible,  notwithstanding  the  fact  that 


370  ELEMENTAKY   PHYSICS 

the  ball  may  have  moved  thousands  of  feet  toward  the 
earth. 

It  is,  scarcely  likely  that  anyone  will  ever  be  able  to 
prove  by  means  of  direct  observations,  that  the  earth 
falls  toward  the  ball.  But  it  has  been  proved  by  as- 
tronomers that  the  earth  and  moon  fall  toward  each 
other,  and  that  the  quantity  of  motion,  when  the  earth 
falls  toward  the  moon,  is  the  same  as  the  quantity  of  mo- 
tion when  the  moon  falls  toward  the  earth.  Why  this 
does  not  cause  the  earth  and  moon  to  rush  toward  each 
other  and  to  break  each  other  up  into  fragments,  cannot 
be  understood,  until  the  character  and  causes  of  curvi- 
linear motion  are  known  (§  345). 

333,  Uniform  Motion. — A  body  which  moves  over  equal 
distances  in  successive  equal  intervals  of  time  is  said  to 
have  uniform  motion.  For  instance,  if  a  body  continues 
to  move  over  a  distance  of  5  feet  each  second,  it  neither 
increases  nor  decreases  in  velocity  and  its  motion  is  uni- 
form. A  body  which  moves  a  distance  of  5  feet  each 
second  will  in  6  seconds  move  5  x  6  =  30  feet.  Hence, 
to  find  the  distance  passed  over  when  the  uniform  veloc- 
ity and  the  time  are  given,  multiply  the  velocity  by  the 
time. 

Calling  the  velocity  v,  and  the  time  t,  and  the  entire  dis- 
tance passed  over  s,  the  above  rule  can  be  stated  in  the 
language  of  mathematics  by  s  —  vt.  Such  a  statement  is 
called  a  formula-  After  the  formula  for  a  given  class  of 
problems  has  once  been  secured,  there  is  no  longer  any 
need  of  going  through  the  long  reasoning  process  re- 
quired either  in  obtaining  the  formula,  or  in  working 
out  each  problem  independently.  All  that  is  required  is 
to  substitute  for  the  letters  in  the  formula  their  respec- 
tive values  as  given  in  the  problem.  This  results  in  an 


MECHANICS  371 

equation  which  can  be  readily  solved.  If  it  is  required 
to  find  the  distance  passed  over  in  10  seconds  by  a  body 
having-  a  uniform  velocity  of  8  feet  per  second,  -substi- 
tute, in  the  formula  s  =  vt,  8  for  v,  and  10  for  t,  and 
the  result  is  s  —  8  x  10.  Whence  s  =  80  feet. 

It  is  often  convenient  to  have  the  formula  so  written 
that  the  quantity  whose  value  is  to  be  found  is  placed  by 
itself  on  one  side  of  the  sign  of  equality.  If  instead  of 
the  above  problem,  it  were  given  that  a  body  moved  80 
feet  in  10  seconds,  to  find  the  time,  the  80  would  be  sub- 
stituted for  s  in  s  =  vt  and  10  for  t,  which  would  give  the 
result  80  =  v  x  10,  or  80  —  lOv  ;  whence  v  =  8.  By  alge- 

o 

braic  processes  s  =  vt  can  be  readily  changed  into  v  =  -» 

o  t 

and  t  =  —    In  the  problem  just  given  it  would  be  more 

convenient  to  use  the  formula  in  the  form  of  t  =  —  than 

v 

in  the  original  form  of  s  =  vt. 

334.  Uniformly  Accelerated  Motion.  —  Anyone  who 
watches  a  train  starting  from  a  station  can  observe  that 
the  velocity  of  the  train  is  constantly  increasing  until  full 
speed  is  attained,  when  its  motion  is  practically  uniform. 
On  the  contrary,  when  a  train  approaches  a  station,  its 
speed  becomes  slower  and  slower  until  the  train  stops. 
If  it  be  desired  to  determine  the  velocity  of  the  train  at 
any  point  while  its  speed  is  either  increasing  or  decreas- 
ing, it  will  be  necessary  to  determine  what  distance  the 
train  would  pass  over  in  a  second,  minuto,  or  hour,  with  a 
uniform  velocity,  equal  to  the  velocity  of  the  train  at  the 
point  in  question.  If  a  ball  rolling  down  hill  has  at  the 
end  of  the  third  second  a  velocity  of  20  feet  per  second, 
it  will  move  during  the  fourth  second  a  distance  of  20 
feet,  provided  its  velocity  does  not  change  after  the  third 
second. 


372  ELEMENTARY   PHYSICS 

A  force,  acting  upon  a  body  which  is  free  to  move,  pro- 
duces motion  in  that  body.  As  long  as  the  force  con- 
tinues to  act  in  the  same  direction,  the  body  will  continue 
to  gain  in  velocity.  The  rate  of  gain  in  velocity  is  called 
acceleration.  If  the  force  is  uniform,  the  acceleration  will 
be  uniform.  If  a  body  started  from  rest  and  had  a 
velocity  of  5  feet  per  second  at  the  end  of  the  first  second, 
a  velocity  of  10  feet  per  second  at  the  end  of  the  second 
second,  and  a  velocity  of  15  feet  per  second  at  the  end  of 
the  third  second,  it  can  be  seen  that  the  rate  of  gain  in 
velocity  per  second  is  5  feet  per  second.  If  a  body  start- 
ing from  rest  has  a  uniform  acceleration  for  each  second 
of  5  feet  per  second,  it  will  have  at  the  end  of  10  seconds 
a  final  velocity  of  10  x  5  =  50  feet  per  second.  Hence  to 
find  the  final  velocity  for  a  body  starting  from  rest,  when 
the  time  and  acceleration  are  given,  multiply  the  accel- 
eration by  the  time. 

Calling  the  final  velocity  v,  the  acceleration  a,  and  the 
number  of  seconds  t,  we  can  express  the  above  rule  by 
the  formula  v  —  at.  From  this  formula  can  be  derived 

111-  V  3    J.  V 

by  algebraic  processes  a  ==  -'  and.  t  =•  -' 

t  a 

If  a  body  start' from  rest  with  a  uniform  acceleration  of 
8  feet  per  second,  it  will  have  a  velocity  at  the  end  of  10 
seconds  of  10  x  8  =  80  feet  per  second.  Since  the  body 
started  from  rest,  its  initial  velocity  is  0,  and,  since  its 
final  velocity  is  80  feet  per  second,  its  average  velocity 
for  the  10  seconds  is  J  of  80  =  40  feet  per  second.  Then 
the  distance  passed  over  is  10  X  40  =  400  feet. 

Calling  the  acceleration  a,  and  the  number  of  seconds 
t,  the  final  velocity  is  at.  The  average  velocity  then  is  \ 

of  at  —  —'      Multiplying    the    average   velocity  by  the 

at  at* 

time  we  have  —  x  t  =  — -  or  J  at\    Hence  the  distance  (s) 

A  2l 


MECHANICS 


373 


passed  over  may  be  represented  by  the  formula  s  =  J  at2. 
That  is,  the  distance  passed  over  by  a  body  starting  from 
rest  with  uniform  acceleration  is  equal  to  one -half  the 
acceleration  multiplied  by  the  square  of  the  time. 

By  algebraic  processes  the  formula  6-  =  \  at2  can  be 
changed  into  the  following  forms : 

2s   .          f2s  v2 

a  =  —'  t  —  \/  — '  and  s  —  — • 

t*  V  a  2a 

335.  Falling  Bodies.— It  has  been  ascertained  by  experi- 
ment that,  if  a  body  is  free  to  fall,  it  will 
acquire  during  each  second  of  its  fall  an 
increase  in  velocity  of  32  feet  per  second 
(32.16  ft.  at  New  York  City).  This  is  true  of 
all  bodies,  whether  large  or  small,  light  or 
heavy,  provided  there  is  nothing  to  resist 
their  fall.  The  reason  a  piece  of  paper  does 
not  fall  as  fast  as  a  lead  bullet,  when  the  two 
are  dropped  simultaneously  from  the  top  of 
a  tower,  is  because  in  proportion  to  its  weight 
the  paper  meets  with  more  resistance  from  the 
air  than  does  the  bullet.  If,  however,  the  two 
had  been  dropped  in  a  vacuum  instead  of  in 
the  air,  they  would  both  have  struck  the 
ground  at  the  same  time  (Fig.  161). 

It  is  true  that  the  farther  we  get  above  the 
earth's  surface,  the  less  is  the  force  of  gravity, 
but,  unless  the  body  under  consideration  is 
falling  from  a  very  great  height,  the  force  of 
attraction  at  any  locality  may  without  pro- 
ducing any  appreciable  error  be  considered 
constant.  Since  this  force  is  constant,  the  ac- 
celeration due  to  gravity  is  uniform  and  the  laws  of  fall- 
ing bodies  are  the  same  as  those  of  uniformly  accelerated 


374  ELEMENTAKY   PHYSICS 

motion.  Instead,  however,  of  representing  the  accelera- 
tion due  to  gravity  by  a,  it  is  customary  to  represent  it  by 
g,  the  initial  letter  of  the  word  gravity.  We  may,  therefore, 
re-write  the  formulas  already  secured  for  bodies  having 
uniformly  accelerated  motion,  so  as  to  make  them  apply 
to  falling  bodies.  Substituting  g  for  a  in  the  formulas 
for  bodies  having  uniformly  accelerated  motion  (§  334), 
we  have  the  formulas  for  falling  bodies,  v  =  gt,  s  =  J  gt2, 

2s  '        A/~2s  v2 

g  =  — ,  t  —  V  — ,  and  s  =  — • 
t*  g  2g 

336.  Method  of  Determining  the  Distance  a  Body  Falls 
during  Any  Particular  Second. — If  it  be  desired  to  find  how 
far  a  body  will  fall  during  the  tenth  second,  find  how  far 
it  will  fall  during  10  seconds,  then  how  far  it  will  fall  dur- 
ing 9  seconds  ;  from  the  first  subtract  the  second,  and  the 
remainder  is  the  distance  it  will  fall  during  the  10th  sec- 
ond.    To  make  this  general,  suppose  it  is  desired  to  find 
the  distance  fallen  during  the  tf-th  second.     The  distance 
fallen  during  t  seconds  is  Jgt 2,  and  during  t  —  1  seconds 
is  ig(t  -  I)2,     igt2  -  |g(t  -  1) 2  =  ig  (2t  -  1),  the  distance 
fallen  during  the  t-th  second.     That  is,  to  find  the  dis- 
tance passed  through  by  a  body  falling  from  rest  during 
any  given  second,  multiply  twice  the  number  of  seconds 
minus  1  by  one-half  of  the  acceleration  due  to  gravity. 

337.  The  Pendulum. — Fasten  a  small  iron  or  lead  ball 
to  the  end  of  a  thread  and  attach  the  other  end  of  the 
thread  to  a  fixed  point  to  serve  as  a  support.     The  thread 
assumes  a  vertical  position,  and  is  in  line  with  the  direc- 
tion in  which  the  force  of  gravity  acts.     Pull  the  ball 
to  one  side  and  let  go.     The  ball  vibrates  to  and  fro. 
This  thread  and  ball  constitute  a  pendulum.     It  can  be 
seen  that  as  the  ball  is  drawn  to  one  side,  it  must  also 
be  raised.    When  the  hand  lets  go  of  the  ball,  the  ball 


MECHANICS  375 

begins  to  descend,  but,  on  account  of  the  string-  it  can- 
not descend  directly  downward,  but  it  descends  as  much 
as  it  can  and  its  path  is  the  arc  of  a  circle  of  which  the 
string  is  the  radius.  "When  the  ball  has  reached  its 
original  position,  it  is  as  low  as  it  can  get,  but,  in  falling 
to  this  position,  the  ball  has  acquired  a  velocity,  and  its 
momentum  carries  it  past  this  lowest  point  and  the  ball 
begins  to  ascend  on  the  opposite  side.  The  ball  con- 
tinues to  ascend  on  the  opposite  side  until  its  motion 
has  all  been  spent,  when  it  again  begins  to  descend  and 
the  operation  is  repeated. 

In  investigating  the  theory  of  the  pendulum,  it  is  con- 
venient to  make  use  of  an  ideal  or  simple  pendulum 
which  may  be  denned  as  a  heavy  material  point  sus- 
pended from  a  fixed  point  about  which  it  is  free  to  move, 
by  a  string  without  weight  and  incapable  of  stretching. 
Of  course,  no  such  pendulum  actually  exists. 

All  pendulums  which  do  not  comply  with  the  conditions 
of  an  ideal  or  simple  pendulum  are  called  compound 
pendulums.  Since  there  can  be  no  string  without  weight 
and  no  material  body  without  size,  all  pendulums  in 
actual  existence  are  compound.  The  nearest  approach 
which  can  be  made  to  a  simple  pendulum  is  to  suspend  a 
small  metal  ball  by  means  of  a  fine  thread. 

338.  Terms  Defined.— A  simple  Vibration  or  Oscillation 
is  the  motion  of  the  pendulum  from  one  extreme  of  its 
swing  to  the  other  extreme.  The  term  vibration  is  fre- 
quently used  to  denote  a  simple  vibration. 

The  Centre  of  /Suspension  is  the  point  of  support  about 
which  the  pendulum  vibrates  (C,  Fig.  162). 

A  Complete  Vibration  is  the  motion  of  the  pendulum 
from  one  extreme  of  its  swing  to  the  other  extreme  and 
back  again  (A  D  A  or  B  D  A  B). 


376 


ELEMENTAKY   PHYSICS 


B 

FIG.  162. 


The  Amplitude  of  Vibration  is  half  the  angle  described 
by  the  swinging  pendulum  (angle  A  C  B  or  B  C  D). 

The  Period  of  Vibration  of  a  pendu- 
lum is  the  time  required  to  make  a 
simple  vibration. 

The  Rate  of  Vibration  is  the  number 
of  simple  vibrations  made  by  the  pen- 
dulum in  one  second. 
»D  339.  The  Rate  of  Vibration  Depends 
upon  the  Length  of  the  Pendulum. — 
Suspend  two  lead  balls  by  means  of 
threads  of  equal  length  so  that  the  balls  will  hang  side 
by  side  without  touching.  Cause  them  to  vibrate  so  that 
the  amplitude  of  one  vibration  is  larger  than  that  of  the 
other.  They  both  require  equal  times  to  make  a  vibra- 
tion. Each  successive  vibration  of  a  pendulum  requires 
the  same  amount  of  time  equal  to 
that  required  by  the  preceding 
one.  If,  however,  the  pendulum 
be  removed  to  a  place  where  the 
force  of  gravity  is  greater,  it  will 
vibrate  faster,  but  if  taken  to  a 
place  where  the  force  of  gravity 
is  less,  it  will  vibrate  more 
slowly. 

Suspend  a  third  ball  by  means 
of  a  shorter  thread  and  cause  it 
to  vibrate.  The  shorter  pendu- 
lum vibrates  faster  than  the 
longer  one.  It  has  been  found 

that  a  pendulum  4  times  as  long  as  another  pendulum 
vibrates  half  as  fast,  and  one  9  times  as  long  vibrates 
only  one-third  as  fast. 


FIG.  163. 


MECHANICS  377 

340.  The  Law  of  the  Pendulum.  —  The  law  of  the  pen- 


dulum is  commonly  expressed  by  the  formula  t  =  w  * 

O 

in  which  -*  —  3.1416,  1  =  length  of  the  pendulum,  and  g— 
the  acceleration  due  to  gravity.  The  development  of 
this  formula  involves  the  use  of  higher  mathematics,  and 
hence  would  be  out  of  place  here.  This  formula  con- 
tains three  quantities,  either  two  of  which  being  known, 
the  third  can  be  found.  Thus  in  any  locality  where  g  is 
known,  the  length  of  a  seconds  pendulum  can  be  com- 
puted, or,  if  the  length  and  g  are  known,  the  time  of 
vibration  can  be  found,  and,  if  the  time  of  vibration  and 
the  length  are  known,  g  can  be  determined.  The  length 
of  a  pendulum  which  beats  seconds,  when  at  sea-level  in 
the  latitude  of  New  York  City,  is  39.1  inches. 

341.  Length  of  Compound  Pendulum.  —  Most  compound 
pendulums  in  use  consist  of  a  ball  or  bob  suspended  by 
means  of  a  rigid  rod.  Since  a  short  pendulum  vibrates 
faster  than  a  long  one,  the  particles  in  its  upper  part  are 
retarded  by  those  below,  and  the  particles  below  are  ac- 
celerated by  those  above.  But  there  must  be  one  point 
which  is  neither  accelerated  nor  retarded,  that  is,  the  ac- 
celeration given  it  by  the  particles  above  is  counteracted 
by  the  retardation  of  the  particles  below  it.  This  point 
is  called  the  centre  of  oscillation.  The  distance  from  the 
centre  of  suspension  to  the  centre  of  oscillation  consti- 
tutes the  theoretical  length  of  the  pendulum. 

It  has  been  ascertained  that  if  a  pendulum  be  sus- 
pended at  its  centre  of  oscillation,  the  rate  of  vibration 
is  not  changed  ;  hence,  the  centre  of  suspension  and  the 
centre  of  oscillation  are  interchangeable.  This  fact 
furnishes  the  means  of  ascertaining  the  length  of  a 
compound  pendulum. 


378 


ELEMENTARY  PHYSICS 


---is 


Fig-.  164  is  that  of  a  reversible  pendulum  with  two 
movable  weights,  M  and  N,  which  can  be  so  adjusted  that 
the  pendulum  requires  the  same  length  of  time  to 
complete  a  vibration  whether  suspended  at  A  or  at 
B.  This  pendulum  vibrates  in  the  same  time  as  the 
pendulum,  S  O,  which  consists  of  an  iron  ball 
suspended  by  a  thread. 

342.  Uses  of  the  Pendulum The 

most  familiar  use  of  the  pendulum 
is  that  of  serving  as  a  regulator  of 
the  motion  of  clocks.  It  also  fur- 
nishes the  most  precise  and  most 
convenient  way  of  measuring  the 
force  of  gravity.  The  first  experi- 
mental proof  which  showed  that  the 
earth  is  flattened  at  the  poles  was 
obtained  by  counting  the  vibrations 
of  a  pendulum  in  different  latitudes. 
The  United  States  Coast  and  Geo- 
detic Survey  uses  the  pendulum 
exclusively  in  all  of  its  gravity 
work.  The  pendulum  used  by  the 
United  States  is  enclosed  in  an  air- 
tight chamber  from  which  the  air 
can  be  exhausted  to  any  press- 
ure desired. 

343.  Inertia — If  a  stone  lie  on  the 
ground,  it  will  remain  in  the  same  po- 
sition until  some    force  causes  it  to 
move    If  a  book  has  been  placed  upon 
a  desk  and  afterwards  is  gone,  it  is 
certain  that  its  removal  was.  due  to  some  force'.  I  The  book 
has  no  power  of  setting  itself  in  motion. 


I  /H\ 

,A     I* 

T 

1 

;m  \ 

d 

3 

JIN 

UN 

a 

o 

1 

HM 

>y<- 

<$Z     o 

If 

A 

—  « 

A 

g 

S] 

— 4-O- 


FIG.  164 


FIG.  1G5. 


MECHANICS  379 

If  a  ball  is  already  in  motion,  force  is  required  to 
change  the  character  of  that  motion.  Force  is  required 
to  make  the  ball  move  faster  or  to  make  it  move  more 
slowly,  or  to  make  it  move  to  one  side,  or  to  stop  it 
entirely. 

When  a  ball  is  moving  over  any  ordinary  surface  and 
force  is  no  longer  applied  to  it,  the  ball  will  soon  come 
to  rest.  The  force  which  resists  or  opposes  the  motion 
in  this  case  is  friction.  If  the  friction  is  made  very  small 
by  securing  a  smooth  surface  for  the  ball  to  roll  upon, 
the  ball  will  roll  for  a  much  longer  time.  The  resist- 
ance of  the  air  also  helps  to  stop  the  ball.  If  it  were 
possible  to  secure  ice  whose  surface  is  perfectly  smooth 
and  flat,  in  a  region  where  there  is  no  air,  a  boy,  moving 
rapidly  along  this  surface,  would  not  be  able  to  stop 
himself.  There  would  be  no  friction,  either  on  the  part 
of  the  ice  or  on  the  part  of  the  air,  to  decrease  the  speed 
of  his  motion. 

Every  body  continues  in  its  state  of  rest  or  in  its  state 
of  uniform  motion  in  a  straight  line  unless  it  is  compelled 
by  some  force  to  change  that  state.  Any  change  which  a 
body  may  exhibit  is  at  once  the  evidence  of  the  existence 
of  some  force.  Force  may  be  described  as  anything 
which  produces,  or  tends  to  produce,  opposes,  or  changes 
motion  in  any  manner.  The  inability  of  a  body  to  start 
to  move  from  a  state  of  rest,  or  to  change  the  character 
of  its  motion  when  it  is  already  in  motion,  is  called 
inertia. 

344.  Units  of  Force, — There  are  two  systems  of  units 
used  in  measuring  force :  the  absolute  units  and  the  grav- 
ity units. 

In  the  gravity  system  the  unit  is  the  pound,  or,  when 
the  metric  system  of  units  is  used,  it  is  the  kilogram  or 


380  ELEMENTARY   PHYSICS 

the  gram.  The  pound  and  the  kilogram  or  gram  are 
the  names  of  units  used  in  measuring  both  mass  and 
force.  By  the  mass  of  a  body  is  meant  the  amount  of 
matter  which  it  contains,  not  its  weight.  By  its  weight 
is  meant  the  force  with  which  it  tends  to  move  to- 
ward the  earth.  Mass  and  weight  are  not  identical. 
The  pound  mass  is  the  mass  of  a  piece  of  platinum 
carefully  preserved  at  the  national  capital.  The  force 
pound  is  the  force  of  the  attraction  which  the  earth 
exerts  upon  a  body  on  its  surface  whose  mass  is  one 
pound.  The  kilogram  is  the  force  of  the  attraction 
which  the  earth  exerts  upon  a  body  having  a  mass  of 
one  kilogram. 

The  mass  of  a  body  remains  constant  wherever  the 
body  may  be  placed,  while  the  attraction  of  the  earth  for 
it  varies  with  the  latitude  and  altitude  of  the  place  where 
the  body  is  located.  A  body  if  weighed  at  the  equator 
would  weigh  less  than  if  weighed  at  the  poles,  and  its 
weight  would  be  less  on  the  summit  of  a  mountain  than 
at  the  base.  If  a  body  having  a  mass  of  one  pound 
should  be  transported  to  the  moon,  and  then  to  the  sun, 
it  would  be  found  that  the  body  is  attracted  on  the  sur- 
face of  the  moon  by  a  less  force,  and  on  the  surface  of 
the  sun  by  a  greater  force  than  on  the  surface  of  the 
earth.  Since  the  force  of  gravity  varies  at  different 
points  on  the  earth's  surface,  the  gravity  unit  also  varies, 
and,  for  this  reason,  it  is  not  suitable  for  precise  scien- 
tific investigations. 

Momentum  has  been  defined  as  the  quantity  of  motion 
of  a  body  and  is  found  by  multiplying  the  mass  of  a  body 
by  its  velocity.  Change  of  momentum  in  a  given  time  is 
proportional  to  the  force  which  acts  ;  therefore,  the  force 
may  be  measured  by  this  change  of  momentum.  A  force 


MECHANICS  381 

which  in  one  second  would  give  a  body  of  ten  pounds 
mass  a  velocity  of  2  feet  per  second,  would  give  a  body 
of  one  pound  mass  a  velocity  of  20  feet  per  second. 

A  force  which,  if  acting-  upon  a  mass  of  one  pound  for 
one  second,  will  produce  a  change  of  velocity  of  one  foot 
per  second,  is  called  a  poundal.  A  force  which,  if  acting 
upon  a  mass  of  one  gram  for  one  second,  will  produce  a 
change  of  velocity  of  1  centimetre  per  second,  is  called  a 
dyne.  These  are  absolute  units  whose  values  are  invari- 
able. 

On  the  earth  a  body  having  a  mass  of  one  pound  is 
said  to  weigh  one  pound.  If  this  body  be  allowed  to  fall 
it  will  acquire  in  one  second  a  velocity  of  32.16  feet  per 
second  (g  at  New  York  City  is  32.16  feet).  Since  the  force 
of  gravity  acting  upon  a  body  of  one  pound  mass  for 
one  second  gives  it  a  velocity  of  32.16  feet  per  second,  a 
force  of  one  pound  must  be  equal  to  32.16  poundals. 
Therefore,  to  change  pounds  to  poundals,  multiply  the 
number  of  pounds  by  32.16,  or  g.  Likewise,  a  body  of  one 
gram  mass,  when  free  to  fall,  acquires  in  one  second  a 
velocity  of  980  centimetres  per  second.  Hence  a  force  of 
1  gram  is  equal  to  980  dynes.  In  each  case  it  will  be  ob- 
served that  one  gravity  unit  is  equal  to  g  absolute  units. 

Let  ^represent  the  force,  m  the  mass  of  the  body,  and 
a  the  acceleration ;  then,  since  force  may  be  measured 
by  change  of  momentum,  which  is  the  mass  multiplied  by 
the  acceleration,  we  have  the  formula  F—  ma  (poundals 
or  dynes). 

345.  Curvilinear  Motion  and  Centrifugal  Force.  —  One 
force  may  cause  a  body  to  move,  but  a  second  force,  act- 
ing not  in  the  same  direction  as  the  first,  is  necessary  to 
cause  the  body  to  move  in  a  curve. 

When  a  stone  is  fastened  to  a  string  and  whirled,  a  part 


382 


ELEMENTARY  PHYSICS 


PIG.  166. 


of  the  force  exerted  by  the  hand  is  used  in  urging-  the 
stone  forward,  while  the  other   part  is  used  in  pulling 

the  stone  toward  the  hand. 
After  the  stone  is  once  set 
in  motion,  very  little  force 
is  required  to  keep  it  mov- 
ing, since  all  of  the  resist- 
ance to  be  overcome  comes 
from  the  air. 

If  the  string,  in  any  po- 
sition, be  cut  by  means  of 
a  very  sharp  knife,  the 
stone  will  start  away  in  a 
straight  line  which  forms 
a  right  angle  with  the  po- 
sition which  the  string 

occupied  the  instant  it  is  cut.     Thus  it  can  be  seen  that 

the  stone  is  continually  being  pulled  in  toward  the  hand, 

in  order  to  keep  it  from 

going  away  in  a  straight 

line.    If  the  direction  in 

which  the  stone  actually 

moves  is  compared  with 

the    straight   path   along 

which    the    stone    would 

move,  if  it  were  not  con- 
tinually pulled  aside,  it  is 

seen  that  the  pull  of  the 

hand     practically    moves 

the  stone  toward  the  hand. 

The  outward  pull  which 

is  experienced   on  whirling  the   stone  about  the  hand 

by  means  of  a  string  is  called  centrifugal  force.     The 


MECHANICS  383 

pull  exerted  by  the  hand  is  known  as  centripetal  force 
and  is  equal  to  and  opposite  in  direction  to  the  centri- 
fugal force. 

Suppose  a  body  to  be  moving-  in  a  circle  of  radius  r 
with  the  uniform  velocity  v,  so  that  in  the  space  of  time  t 
it  will  have  gone  from  A  to  E.  Then  arc  A  E  =  vt.  In 
passing  from  A  to  E  the  body  has  been  drawn  aside  from 
the  straight  path  A  D  toward  the  centre  C,  a  distance  equal 
to  D  E  (or  A  B).  A  uniform  force  tends  to  give  a  body 
uniformly  accelerated  motion  (§  334).  Therefore,  since 
the  force  pulling  the  body  toward  the  centre  is  uniform, 
the  acceleration  given  this  body  toward  the  centre  must 
be  uniform. 

A  B  =  the  distance  the  body  has  been  drawn  aside  from 
the  straight  path  A  D  in  time  t.  Therefore  A  B  =  \  at 
(§  334).  If  t  is  taken  very  small,  the  difference  in  length 
between  the  chord  A  E  and  the  arc  A  E  becomes  too 
small  to  be  readily  recognized ;  the  chord  A  E  and  the  arc 
A  E  may,  for  purposes  of  this  discussion,  be  considered 
identical.  By  geometry  it  can  be  proven  that  the  tri- 
angles AEG  and  ABE  are  similar. 

/.  A  G  :  A  E  ::  A  E  :  A  B. 

A  G,  being  the  diameter,  is  equal  to  2r.  A  E,  the  dis- 
tance passed  over  by  the  body  with  uniform  velocity  v,  is 
equal  to  vt.  A  B,  the  distance  moved  due  to  the  ac- 
celeration a  toward  the  centre,  is  equal  to  \  a&.  Sub- 
stituting these  values  in  the  above  proportion, 

2r  :  vt  :  :  vt  :  \  at\ 
Whence  v2t2  =  \  at?  x  2rt 

v* 
and  a  =  — 

v*  T 

Substitute  —  for  a  in  formula  F  =  Mat 

T 

I  -mr   9 

Centrifugal  force,  or  F.—  — 


384  ELEMENTARY   PHYSICS 

346.  Work. —  Work  is  the  overcoming  of  resistance  in 
producing  or  maintaining  motion.  The  amount  of  work 
which  a  force  (F)  acting  upon  a  body  has  accomplished, 
depends  entirely  upon  the  distance  (s)  the  force  has  act- 
ually moved  the  body  in  opposition  to  some  resisting 
force  ( W  —  Fs).  A  force  may  act,  yet  do  no  work,  pro- 
vided no  motion  results. 

The  unit  of  work  used  by  English-speaking  engineers 
is  the  foot-pound,  which  is  the  work  done  in  moving  a 
body  one  foot  against  a  resistance  of  one  pound.  It  is 
best  illustrated  as  being  the  work  done  in  lifting  a  pound 
weight  through  a  vertical  distance  of  one  foot.  All  re- 
sistance to  be  overcome,  however,  does  not  consist  in 
lifting  weights  against  gravity.  Work  is  done  when  a 
board  is  torn  loose  from  the  side  of  a  house,  or  in  draw- 
ing a  body  horizontally  over  the  surface  of  another 
body. 

The  kilogram-metre  is  the  work  done  in  moving  a  body 
one  metre  against  a1  resistance  of  one  kilogram. 

The  absolute  units  of  work  are  the  foot-poundal  and  the 
erg. 

The  foot-poundal  is  the  work  done  in  moving  a  body 
one  foot  against  a  resistance  of  one  poundal. 

The  erg  is  the  work  done  in  moving  a  body  one  centi- 
metre against  a  resistance  of  one  dyne. 

Power. — Power  in  mechanics  is  the  rate  at  which  an 
agent  can  work.  The  horse-power  is  the  unit  of  power 
and  is  that  power  which  can  do  33,000  foot-pounds  of 
work  in  one  minute,  or  550  foot-pounds  in  one  second. 
The  horse-power  was  introduced  by  Watt  so  that  the 
values  of  the  engines  could  be  compared  with  the  work 
of  horses.  A  horse  cannot  work  quite  at  this  rate,  but  a 
margin  was  allowed  to  prevent  the  mistake  of  making  a 


MECHANICS  385 

horse-power  too  small.     A  man  can  work  at  the  rate  of 
a  little  less  than  \  of  a  horse-power. 

347,  Energy. — Energy  is  the  ability  to  do  work.    Boys 
have  learned  that  an  apple  hanging  beyond  their  reach 
on  a  tree  can  be  brought  to  the  ground  by  a  stone  or  club. 
The  moving  stone  possesses  the  ability  to  tear  the  apple 
loose  from  the  tree  and  gravity  then  causes  the  apple 
to  fall.     The  stone  possesses  the  ability  to  do  work  be- 
cause of  its  motion. 

If  a  string  be  tied  to  a  stone  lying  on  a  shelf  and  the 
string  be  passed  over  a  pulley  and  fastened  to  a  lighter 
object  on  the  floor  below,  the  stone,  when  free  to  fall,  will 
raise  the  body  from  the  floor.  The  stone  by  virtue  of  its 
position  up  on  the  shelf  is  able  to  perform  work. 

There  are  two  types  of  energy,  kinetic  and  potential. 
Kinetic  energy  is  the  energy  of  a  body  by  virtue  of  its  mo- 
tion. Potential  energy  is  the  energy  of  a  body  by  virtue 
of  its  position  in  reference  to  other  bodies,  or  of  the 
relative  position  of  its  parts.  Any  body  elevated  above 
the  surface  of  the  earth,  a  coiled  or  bent  spring,  or  any 
elastic  body  changed  from  its  natural  shape  possesses 
potential  energy.  In  fact,  a  body  possesses  potential 
energy  if  any  condition  is  present  which  is  capable  of 
producing  motion  in  that  body,  such  as  strain,  gravita- 
tive  separation,  chemical  separation,  and  electrification. 

348.  How  Energy  is  Measured,— Energy  can  be  measured 
only  by  the  work  it  can  do,  therefore  the  units  of  energy 
are  the  same  as  the  units  of  work. 

Suppose  a  body  of  a  mass  of  m  pounds  start  vertically 
upward  with  a  velocity  of  v  feet  per  second,  then  the 

9 

distance  (s)  it  will  rise  is  —  •     The  work  done  in  raising 

.  v2 

a  mass  of  m  pounds  s  feet  is  ms  foot-pounds.     But  s—  — 


386  ELEMENTARY   PHYSICS 

(§335).      Multiplying-   both   members    of    this   equation 

by  m,  we  have  ms  (the  kinetic  energy  of  the  mass)  —  —J 

2g 

rto/i  ri\% 

Therefore  representing  the  kinetic  energy  by  E,E  =  — 
foot-pounds. 

It  has  been  shown  that  when  a  constant  force  acts  upon 
a  body  with  mass  m  producing  acceleration  a,  that  F  = 
Ma  poundals  or  dynes.  Kepresenting  the  distance  by  s, 
the  work  done  is  Fs  foot-poundals  or  ergs.  But  s  — 

*??  ^  QJ 

—  *     Multiplying  s—  —\>yF  —  Ma,  member  by  member, 


we  have  fs   (the  kinetic    energy  of  the  mass)  =—  — 

A 

Hence  E—\  mrf  foot-poundals  or  eggs. 

349.  Conservation  of  Energy.—  If  a  body  be  projected 
vertically  upward  against  gravity,  it  has  kinetic  energy 
sufficient  to  lift  its  weight  a  certain  height  depending  on 
the  strength  of  the  projecting  force.  As  the  body  rises, 
its  kinetic  energy  is  expended  in  overcoming  the  resist- 
ance of  gravity,  until  when  it  comes  to  rest,  the  whole  of 
its  kinetic  energy  has  been  expended,  or  rather  changed 
into  potential  energy.  By  virtue  of  the  work  done  on 
the  body  during  its  ascent,  it  has,  at  the  instant,  of  com- 
ing to  rest,  an  amount  of  potential  energy  equal  to  the 
amount  of  kinetic  energy  expended  in  lifting  it  to  the 
height  attained.  It  then  falls  with  uniformly  accelerated 
motion,  changing  the  potential  energy  into  kinetic,  and 
acquires  more  and  more  kinetic  energy,  until,  at  the  in- 
stant it  reaches  the  point  from  which  it  was  projected,  it 
has  the  same  amount  of  kinetic  energy  as  when  it  started 
up  —  that  is,  an  amount  sufficient  to  lift  its  own  weight  to 
the  height  from  which  it  fell. 

At  the  beginning  of  the  ascent,  and  at  the  end  >f      '> 


MECHANICS  387 

descent,  the  body  had  the  same  amount  of  kinetic  en- 
ergy ;  at  the  end  of  the  ascent,  and  the  beginning  of  the 
descent,  it  had  only  potential  energy. 

At  any  point  of  the  ascent  it  had  enough  potential 
energy  to  lift  it  to  the  height  already  attained,  and 
enough  kinetic  energy  to  lift  it  the  remainder  of  the 
height  yet  to  be  attained.  At  any  point  of  the  descent  it 
had  enough  kinetic  energy  to  lift  it  back  to  the  point  from 
which  it  started  to  fall,  and  enough  potential  energy  to 
lift  it  from  the  lowest  point  of  fall  to  its  present  position. 
The  sum  of  kinetic  and  potential  energies  remains  con- 
stant. 

A  body  in  motion  on  the  earth's  surface  will  sooner  or 
later  come  to  rest  after  the  propelling  force  ceases  to  act, 
because  the  motion  of  every  body  on  the  earth  is  opposed 
by  friction.  What  has  become  of  the  energy  of  the  mov- 
ing body  ?  It  has  been  transformed  into  heat.  A  bullet, 
fired  against  a  stone,  is  stopped  by  the  stone.  The  bullet 
is  warmed  by  its  impact  against  the  stone. 

When  a  mechanical  engineer  examines  a  dynamo  for 
the  first  time,  he  cannot  understand  why  so  much  driv- 
ing power  is  needed  to  make  the  armature  rotate.  He 
pees  th.-.t  the  friction  of  the  bearings  and  of  the  brushes 
agai1  st  the  commutator  is  so  small  that  it  can  absorb 
but  a  small  part  of  the  energy  delivered  by  the  engine. 
Some  cause  other  than  friction  alone  must  be  looked  for. 
It  is  found  in  the  fact  that,  when  the  wires  of  the  arma- 
ture cut  the  lines  of  magnetic  force,  a  resistance  or  drag 
is  experienced  by  the  wires,  and  the  energy  required  to 
overcome  this  drag  is  converted  into  the  energy  of  an 
electric  current. 

heat  is  a  form  of  energy  can  be  seen  in  a  locomo- 
drawing  a  train  of  cars.     Here  it  is  the  heat  from  the 


388  ELEMENTARY   PHYSICS 

burning-  coal  that  supplies  the  engine  with  the  required 
energy. 

That  an  electric  current  is  a  form  of  energy  can  be 
seen  in  any  city  when  an  electric  car  is  passing  down  the 
street.  In  the  locomotive  it  is  the  heat,  in  the  electric 
car  it  is  the  electric  current  which  is  transformed  into 
mechanical  motion. 

Energy  which  is  apparently  lost  is  merely  transformed 
into  some  other  form.  When  it  is  transformed  into  heat, 
the  heat  may  be  radiated  away  into  space  and  become 
lost  or  dissipated,  but  it  cannot  be  annihilated.  Neither 
can  energy  be  created.  This  can  be  summed  up  as  one 
of  the  greatest  conceptions  of  modern  science,  namely 
the  conservation  of  energy.  Energy  can  be  transformed 
from  one  form  into  another,  but  the  sum  total  of  all  the 
energy  in  the  universe  remains  constant. 

350.  Machines. — Of  all  animals  man  alone  makes  use 
of  devices  for  the  advantageous  application  of  force. 
Such  devices  are  known  as  machines.  Machines  are  so 
designed  as  to  employ  force  for  the  attainment  of  a  de- 
sired result.  By  their  use  man  can  do  work  which  other- 
wise would  be  impossible.  He  is  also  thus  enabled  to 
employ  forces  of  nature,  such  as  the  force  of  the  wind, 
the  energy  of  running  or  falling  water,  the  energy  in  fuel, 
and  the  strength  of  other  animals.  Machines  have  been 
devised  for  utilizing  even  the  force  of  the  tides  and  the 
heat  of  the  sun. 

The  force  which  is  applied  to  a  machine  is  called  the 
power;  and  the  resistance  to  be  overcome  is  called  the 
weight.  You  cannot  get  more  work  out  of  a  machine 
than  is  put  into  it,  because  energy  cannot  be  created.  In 
reality  there  is  no  machine  in  which  there  is  no  friction, 


MECHANICS  389 

and,  as  a  result,  no  machine  turns  out  as  much  useful  work 
as  is  put  into  it.  Some  of  the  energy  is  transformed  into 
heat  which  is  radiated  away,  and,  as  far  as  utility  is  con- 
cerned, may  be  considered  wasted.  The  ratio  of  the  useful 
work  done  by  the  machine  to  the  total  work  done  upon 
the  machine  is  known  as  the  efficiency  of  the  machine. 

To  find  the  work  done  by  the  power,  it  is  necessary  to 
multiply  the  power  by  the  distance  through  which  it 
moves.  To  find  the  useful  work  done,  multiply  the 
weight  by  the  distance  through  which  it  is  moved.  If 
a  machine  could  be  made  in  which  there  were  no  friction, 
we  should  have  a,  perfect  machine.  In  such  a  machine  the 
work  done  by  the  power  is  equal  to  the  work  done  upon 
the  weight.  In  the  discussion  of  machines  they  will  be 
considered  as  perfect.  Hence,  we  can  deduce  the  gen- 
eral law  of  machines :  The  power  multiplied  by  the  dis- 
tance through  which  it  moves  is  equal  to  the  weight  multiplied 
by  the  distance  through  which  it  is  moved. 

If  the  machine  under  consideration  has  any  parts  which 
bear  the  same  ratio  to  each  other  as  the  ratio  of  the  dis- 
tance through  which  the  power  moves  to  the  distance 
through  which  the  weight  is  moved,  the  power  and  the 
weight  can  be  respectively  multiplied  by  the  dimensions 
of  these  parts.  For  illustration,  if  both  the  power  and 
the  weight  move  in  circles,  the  power  multiplied  by  the 
radius  (or  diameter)  of  its  circle  is  equal  to  the  weight 
multiplied  by  the  radius  (or  diameter)  of  its  circle. 

From  the  general  law  of  machines  it  can  be  seen  that 
the  faster  the  power  moves  as  compared  with  the  weight, 
the  greater  is  the  weight  which  can  be  moved>  Hence, 
greater  force  on  the  weight  can  be  had  at  the  expense  of 
speed,  while  greater  speed  can  be  obtained  at  the  e^- 
pense  of  force.  In  practice,  when  the  weight  is  a  body 


390  ELEMENTARY   PHYSICS 

moved  against  resistance,  a  little  additional  power  is  re- 
quired to  impart  motion  to  the  body  and  also  to  the  parts 
of  the  machine  which  partake  of  motion.  After*  the  re- 
quired speed  has  been  established,  then  the  power  and 
weight  bear  the  relation  to  each  other  as  suggested  in 
the  general  law  of  machines. 

351.  Simple  Machines.  —  Every  machine,  however  com- 
plicated it  may  be,  is  but  a  combination  of  one  or  more 

of    the    simple    ma- 
^^^^^--^^^i^  •^'^^^^^^^       chines,  or  mechanical 

+  ^^^~  m  Weight  arm  * 

p  power  Arm  F  w     powers.      There    are 

»    Lever  of  the  first  class.  ,  . 

FlQ  168.  six  simple  machines 

—  the  lever,  the  wheel 

-P  and  axle,  the  pulley, 

Power  Arm  . 

|^^^^  the    inclined    plane, 


the    screw,   and    the 

Lever  of  the  second  class. 


352.    The    Lever.- 
The  lever  is  a  rigid 

t    Power  Arm          bar     free    to    move 

^^^^^^ 

««««r<_^      about  a  fixed   point 


w  Weight  arm  ™  . 

Lever  of  the  third  class.  OT  aX1S  Called  l 

FIG.  170  crum.    The   part    of 

the  lever  between  the 

power  and  the  fulcrum  is  called  the  power  arm.  The  part 
of  the  lever  between  the  weight  and  the  fulcrum  is  called 
the  weight  arm. 

There  are  three  classes  of  levers.  A  lever  of  the  first 
class  is  one  in  which  the  fulcrum  is  between  the  weight 
and  the  power  In  a  lever  of  the  second  class  the  weight 
is  between  the  power  and  the  fulcrunie  In  a  lever  of  the 
third  class  the  power  is  between  the  weight  and  the  ful- 
crum. 


MECHANICS 


391 


FIG.  171. 


353.  Law  of  the  Lever.— The  power  multiplied  ly  its  dis- 
tance from  the  fulcrum  is  equal  to  the  iveight  multiplied  l)y 
its  distance  from  the 
fulcrum. 

In  the  case  of  the 
lever,  the  power  and 
the  weight  move  in 
circles  with  the  ful- 
crum as  the  centre,  the 
power  arm  being-  the  radius  of  the  circle  whose  circum- 
ference is  traversed  by  the  power,  and  the  weight  arm 
being  the  radius  of  the  circle 
whose  circumference  is  traversed 
by  the  weight.  Since  the  ratio  of 
the  radii  of  these  circles  is  the 
same  as  the  ratio  of  their  circum- 
ferences, it  is  clear  that  the  law  of 
the  lever  is  derived  from  the  gen- 
eral law  of  machines. 

354.  Wheel  and  Axle.— The  wheel 
and  axle  consist  of  a  wheel  fast- 
ened to  an  axle  so  that  they  turn 
together.     The 
power  is  applied 
to  the  circumfer- 
ence of  the  wheel,  and  the  weight  is  fast- 
ened by  means  of  a  cord  to  the  circum- 
ference of  the  axle. 

The    power    passes    over    a    distance 
equal  to  the  circumference  of  the  wheel 
in  the  same  time  that  the  weight  passes 
over  a  distance  equal  to  the  circumference  of  the  axle. 
Hence,  the  power  multiplied  Ijy  the  circumference  (or  radius 


FIG.  172. 


FIG.  173. 


392 


ELEMENTARY  PHYSIOS 


FIG.  174. 


or  diameter)  of  the  wheel  is  equal  to  the  weight  multiplied 
by  the  circumference  (or  radius  or  diameter]  of  the  axle. 

Instead  of  a  wheel  a  crank 
may  be  used,  as  in  a  windlass. 
Sometimes  the  wTheel  and  the 
axle  do  not  have  the  same  axis, 
as  when  motion  is  communi- 
cated from  wheel  to  wheel  by 
means  of  belts  or  cogs. 

355.  The  Pulley.— A  pulley 
consists  of  a  wheel  free  to  turn 
about  an  axis.  The  wheel  is 

usually  placed  within  a  frame  called  the  block.  If,  while 
the  power  is  acting,  the  block  is  stationary,  the  pulley  is 
said  to  be  fixed;  if  the  block  moves,  the  pulley  is  said 
to  be  movable.  The  rope  or  string  which  passes  over  the 
wheel  of  the  pulley  has  the  same  tension  at  every  point. 

In  a  single  fixed  pulley  (Fig.  175),  the  weight  and 
power  move  equal  distances ;  hence  the  power  is  equal  to 
the  weight.  There  is  no  gain  of  either  force  or  speed, 
but  merely  a  change  of  direction  of  the  force  applied. 

If  a  continuous 
cord  pass  about  a 
movable  and  a  fixed 
pulley  (Fig.  177), 
the  movable  pul- 
ley sustaining  the 
weight  is  supported 
by  two  parts  of  the 
cord,  and  the  power 
must  pass  through 

twice  the  distance  passed  over  by  the  weight.  The 
weight,  therefore,  is  equal  to  twice  the  power.  If  the 


> 


FIG.  175. 


FIG  17G. 


MECHANICS 


393 


cord  pass  about  one  movable  and  two  fixed  pulleys,  the 
pulley  sustaining  the  weight  is  supported  by  three  parts 
of  the  cord  and  the  power  must 
pass  through  three  times  the 
distance  passed  over  by  the 
weight.  Therefore,  the  weight 
is  equal  to  three  times  the 
power.  In  a  similar  manner  it 
can  be  shown  that,  when  one 
continuous  cord  is  used,  the 
power  must  pass  over  a  dis- 
tance as  many  times  greater 


FIG.  177. 


7  than  the  distance  passed  over  by 

£    the  weight,  as  there  are  parts  of  the 

cord  sustaining  the  weight.  Hence, 
the  law  of  the  pulley:  The  iveight 
is  equal  to  the  power  multiplied  l)y 
the  number  of  parts  of  the  cord  sus- 
taining the  weight. 

356.  Inclined  Plane — Any  surface 
inclined  to  a  horizontal  surface  is 
an  inclined  plane.  In  rolling  a 
barrel  into  a  wagon,  it  is  much 
easier  to  roll  the  barrel  on  a  plank 
having  one  end  on  the  ground  and 
the  other  end  resting  on  the  wagon 
so  as  to  form  an  inclined  plane, 
than  it  is  to  lift  the  barrel  vertical- 
ly into  the  wagon.  A  part  of  the 
weight  of  the  barrel  produces  press- 
ure on  the  plank,  while  the  rest  of 
its  weight  tends  to  make  it  roll  down  the  plank  to  the 
ground.  All  the  force  which  is  required  is  that  needed 


FIG.  178. 


394 


ELEMENTARY  PHYSICS 


FIG.  179. 


W 


to  overcome  the  tendency  of  the  barrel  to  roll  down  the 
inclined  plane.    jThe  steeper  the  plank  the  greater  must 
be  the  force  required  to  roll  the  barrel  into  the  wagon.) 
If  the  plank  is  in  a  vertical  position  this  force  must  be 
equal  to  the  weight  of  the  barrel. 

In  Fig.  179,  A  C  is  the 
length  of  the  plane,  B  C  the 
vertical  height,  A  B  the 
base,  a  the  angle  of  the 
plane. 

In  the  inclined  plane  the 
power  must  move  over  the 
entire  length  of  the  plane, 
while  the  weight  is  raised 
the  vertical  distance  from 
fche  foot  of  the  plane  to  the 
top.  Hence,  the  power  multi- 
plied by  the  length  of  the  plane 
is  equal  to  the  weight  multi- 
plied by  the  vertical  lieight  of 
the  plane. 

FIG.  isi.  If  the  power  acts,  in  a  line 

parallel  with  the  base,  the 

power  must  act  over  a  distance  equal  to  the  length  of  the 
base  of  the  plane  while  the  weight  is  raised  the  vertical 
distance  from  the  foot  of  the  plane  to  the  top.  Hence, 
when  the  power  is  applied  in  a  line  parallel  to  the  base,  the 
power  multiplied  by  the  base  is  equal  to  the  weight  multiplied 
by  the  vertical  height  of  the  plane. 

357.  The  Wedge If  it  is  desired  to  raise  one  end  of  a 

stone  block,  this  can  easily  be  accomplished  by  forcing 
an  inclined  plane  under  that  end  of  the  block,  as  shown 
in  Fig.  182.  This  is  the  principle  of  the  wedge,  which 


FIG.  180. 


W 


MECHANICS 


395 


has  usually  the  form  of  a  double  inclined  plane  with  the 
bases  placed  next  to  each  other.  The  law  of  the  inclined 
plane,  where  the  power  is  applied  par- 
allel to  the  base,  applies  also  to  the 
wedge ;  but,  since  the  wedge  is  usually 
driven  by  percussion,  it  is  difficult  to 
apply  any  law  other  than  that  the 
smaller  the  angle  of  the  planes  the 
greater  is  the  weight  which  can  be 
overcome. 
The  wedge  is  of  great  service  in 


FIG.  182. 


FIG.  183. 


splitting  wood  and  in  raising  great  weights  short  dis- 
tances, such  as  raising  ships,  separating  layers  of  rock, 
etc. 

358.  The  Screw, — The  screw  consists  of  a  cylinder  with 


Pitch 


Pitch 


FIG.  181. 


a  rib  or  thread  passing  spirally  around  it.     The  principle 
involved  is  that  of  the  inclined  plane,  as  can  be  shown  by 


396 


ELEMENTARY   PHYSICS 


cutting-  a  piece  of  paper  in  the  shape  of  a  right  triangle 
and  wrapping  it  around  a  lead  pencil  (Fig.  185).  The 
hypotenuse  of  the  triangle  traces  out  the  path  of 
the  thread.  The  distance  between  two  contiguous 
turns  of  the  thread  measured  along  the  axis  of  the 
cylinder  is  known  as  the  pitch  of  the  screw.  The 
screw  works  in  a  block  on  the  inside  of  which  are 
threads  corresponding  to  those  of  the  screw.  This 
block  is  called  the  nut. 

The  power  is  usually  applied  to  a  wheel  or  lever 
fastened  to  the  upper  part  of  the  screw.  In  order 
to  turn  the  screw  once,  the  power  must  pass  over  a 
distance  equal  to  the  circumference  of  the  wheel  or 
the  circle  of  which  the  lever  is  the  radius.  In  one 
turn  of  the  screw  the  weight  is  raised  a  distance 
equal  to  the  pitch  of  the  screw.  Hence,  the  power 
multiplied  by  the  circumference  of  the  wheel  is  equal 
to  the  weight  multiplied  by  the  pitch  of  the  screw. 

359.  The  Great  Importance   of  Mathematics  in 
Modern   Physics.— Modern    Physics    rests 
upon  the  mathematical  development  of 
the  ideas  of    momen- 

FiG.  185. 

turn,  force,  energy, 
work,  power,  and  other  similar 
quantities.  While  it  is  possible 
to  get  an  insight  into  many 
physical  and  chemical  prob- 
lems without  their  use,  they 
are  the  foundations  upon  which 
must  be  built  all  further  prog- 
ress. The  mathematical  de- 
velopment of  these  ideas  is,  therefore,  the  first  care  of 
any  larger  work  011  physics-  It  is  hoped  the  more  ele- 


PlG.  186. 


MECHANICS  397 

mentary  work  here  presented  will  encourage  students  to 
undertake  the  more  advanced  work  based  upon  this 
mathematical  foundation. 


EXERCISES 

1.  Is  it  possible  ever  to  make  a  perpetual  motion  machine  ? 

Why? 

2.  If  a  stone  be  dropped  from  the  top  of  a  high  monument,  the 

stone  will  fall  a  little  to  the  east  of  the  vertical  line. 
Explain. 

3.  Why  cannot  a  man  lift  himself  up  by    stepping  into  a 

bushel  basket  and  lifting  on  the  handles  ? 

4.  Why  is  a  bicyclist  apt  to  fall  if  he  attempts  to  turn  a 

corner  rapidly  when  the  street  is  slippery  ? 

5.  When  a  train  starts,  a  passenger  can  feel  his  body  pressing 

harder  against  the  back  of  the  seat.     Explain. 

6.  When   a  marksman  shoots    at  a  target,   does   the   bullet 

travel  in  a  straight  or  curved  line  ?     Explain. 

7.  If  a  ball  be  fired  from  a  cannon  pointing  vertically  upward, 

will  it  fall  back  into  the  mouth  of  the  cannon  ? 

8.  How  can  a  driver  make  it  easier  for  his  horses  to  draw  a 

heavy  load  up  hill  ? 

9.  If  a  large  hole  were  made  extending  from  one  side  of  the 

earth  through  the  centre  to  the  opposite  side  and  a  ball 
were  dropped  into  this  hole,  what  would  become  of  the 
ball? 

10.  Why  does  the  mud  fly  off  of  your  shoe  when  you  kick? 

11.  Why  do  fast  revolving  grindstones  often  break  ? 

12.  Why  is  more  coal  consumed  in  running  a  train  a  given  dis- 

tance in  which  numerous  stops  are  made  than  is  con- 
sumed by  the  train  in  running  the  same  distance  with- 
out stops  ? 

13.  Why  is  it  so  difficult  to  hold  out  a  heavy  weight  at  arm's 

length  ? 

14.  What  effect  has  the  placing  of  a  live  fish  in  a  bucket  even 

full  of  water  upon  the  weight  of  the  bucket  and  its  con- 
tents? 


398  ELEMENTARY   PHYSICS 

15.  A  stone  dropped    into  a  well    strikes  the   bottom  in  3 

seconds.     How  deep  is  the  well  ? 

16.  A  man  can  pump  200  Ibs.  of  water  per  minute  to  a  height 

of  16  feet ;  how  many  foot-pounds  of  work  does  he  do 
in  an  hour  ? 

17.  A  twenty  H.  P.  engine  is  employed  to  pump  water  from 

the  bottom  of  a  mine  400  feet  deep.  How  many  cu- 
bic feet  of  water  will  it  raise  in  24  hrs.  ?  (1  cu.  ft.  of 
water  =  62£  Ibs. ) 

18.  What  force  is  required  to  roll  a  ball  weighing  300  Ibs.  up 

an  incline  having  a  vertical  rise  of  10  ft.  to  every  100 
ft.  of  the  incline,  when  the  force  is  applied  parallel  to 
the  plane  ?  When  it  is  applied  parallel  to  the  base? 

19.  How  far  will  a  body,  starting  from  rest,  fall  during  the 

first  second  ?    During  the  eighth  second  ? 

20.  What  is  the  kinetic  energy  of  a  stone  of  10  Ibs.  mass  mov- 

ing with  a  velocity  of  15  ft.  per  sec.  ?  What  is  the  mo- 
mentum of  the  stone  ? 

21.  What  weight  can  be  raised  by  a  force  of  100  Ibs.  by  means 

of  one  fixed  and  one  movable  pulley  ? 

22.  What  is  the  length  of  a  pendulum  beating  half  seconds  at 

a  place  where  the  seconds  pendulum  is  39.1  inches? 

23.  The  pitch  of  a  screw  is  ^  inch.     The  length  of  the  lever 

used  in  turning  the  screw  is  2  ft.  What  power  must  be 
applied  to  produce  a  pressure  of  one  ton  ? 

24.  The  radius  of  the  wheel  is  2  ft.  and  of  the  axle  4  in.    What 

weight  can  be  raised  by  a  power  of  50  Ibs.  ? 

25.  A  body  of  mass  2  Ibs.  is  attached  to  the  end  of  a  string  a 

yard  long,  and  is  whirled  round  at  an  uniform  rate, 
making  twenty  revolutions  in  a  minute.  What  is  the 
tension  in  the  string? 

26.  A  force  of  30  dynes  acts  for  10  seconds  upon  a  body  resting 

on  a  smooth  horizontal  plane,  and  imparts  to  it  a  veloc- 
ity of  100  centimetres  per  second.  What  is  the  mass  of 
the  body  ?  ' 


INDEX 


ABSOLUTE  UNITS,  381,  384 

Absolute  zero,  139 
Absorption,  of  ether  waves  by  atoms, 

242 
of  heat  during  change  of  state,  115, 

116 
of  light,  its  relation  to  reflection,  274 

to  transmission,  269-275 
of  radiant  heat,  its  relation  to  radi- 
ation, 279 
to  reflection,  276 
to  transmission,  275-278 
Acceleration.  372 

due  to  gravity,  373 
Acid,  definition  of,  205 
effect  of  on  litmus,  82 
Adhesion,  48 
Affinity,  chemical,  159 
Air,  density  of,  17-19 
transmits  sound,  217 
but  not  radiant  heat  or  light,  237, 

243 

chamber,  36 

gap  in  magnetic  circuit,  321 
gap  in  Ruhmkorff  coil,  357 
pressure,  5-15 
pump,  28,  29 
mercury  air  pump,  239 
liquid,  139 
Albumen,  83 
Alcohol  lamp,  50 
Alkalis,  206 

effect  of  on  litmus,  82 
Alphabet,  code  in  telegraphy,  322 
Alternating  currents,  339-341,  347,  351, 

357 
discharge  across  spark  gap,  358 


Altitude,   determined    by  barometer, 
15 

determined    by    boiling    point     of 

water,  109 
Alum,  96,  276,  278 
Aluminum,  173 
Ammonia  water,  82 
Amplitude  of  vibration,  215,  376 
Analysis,  chemical,  160 

of  color  by  prism,  254,  267,  269 

of  sound,  229 
Aneroid  barometer,  16 
Animals  distinguished    from    plants, 

176 

Animal  body,  origin  of  heat  in,  212 
Antidotes,  207 
Archimedes'  Principle,  71 
Arc  light,  356 

Armature,  320,  324,  328,  336,  339 
Atmosphere,  attracted  by  earth,  367 

chemical  composition  of,  151 

height  of,  238 

pressure  of,  at  sea-level,  14 

total  pressure  on  human  body,  19 
Atomic  theory,  193,  194 
Atoms,  explain  multiple  proportion  in 
compounds,  194 

weight  of,  196 
Attraction,  capillary,  50 

electric,  301 

magnetic,  288-291 

BAROMETER,  aneroid,  16 
mercurial,  15 

used  to  measure  elevations,  15 
to  predict  changes  of  weather,  1 6 

Base  in  chemistry,  205 


403 


404 


INDEX 


Battery,  see  Cell 

Beam  of  light,  243 

Bell,  electric,  338 

Bichromate  cell,  313,  314 

Black,  color,  267 

Boiling  point,  104 
effected  by  pressure,  108 
used  to  determine  elevation,  109 

Boyle's  Law,  21-24 

Brass,  173 

Bronze,  173 

Brushes  of  dynamo,  341 

CALORIE,  112 
Camphor,  87 
Capacity  for  heat,  125 
Capillarity,  50 

cause  of,  53 
Capillary  tubes,  49,  50 
Carbon  dioxide,  2,  147-149,  152,  177 
Caustic  potash,  82 

soda,  191 
Cell,  electric,  311 

connected  in  parallel,  313 
in  series,  326 

copper  sulphate,  crowfoot,  or  grav- 
ity, 312 

potassium  bichromate,  313 

sal  ammoniac,  312 

terminals  -f-  and  —  313 
Centimetre,  399 
Centre,  of  oscillation,  377 

of  suspension,  375 
Centrifugal  force,  382,  383 
Centripetal  force,  383 
Charge  of  electricity,  305 
Chemical    action    as    source  of    elec- 
tricity, 310 

action  due  to  light,  279 

affinity,  159, 194 

analysis,  160 

combination,  178 

double  decomposition,  180 
Circuit,  electric,  313 

earth  a  part  of,  326 

magnetic,  297,  298,  315 

primary  and  secondary,  346 


Clouds,  formation  of,  125 

Code  used  in  telegraphy,  322 

Coherer,  359 

Cohesion,  48 

Coil,  in  dynamo,  336 

induction  coil,  347,  351,  356 
Collecting  rings,  341 
Colorless  transparent  pbjects,  268 
Colors,  black,  267 

compound,  254,  267 

depend    on   rate    and    intensity  of 
vibration  of  atoms,  257-259 

intensity  of,  256-259,  286 

of  flames  and  other  sources  of  light, 
252-257,  264 

of  non-luminous  objects  depend  on 
colors  received  from   luminous 
objects,  265-267,  270 
on  colors  absorbed,  272-275 
on  colors  reflected,  265-367 
on  colors  transmitted,  269-273 

effect  of  thickness,  272 

of  opaque  objects,  365-267,  273 

of  pigments,  274 

of  transparent  objects,  269-272 

prismatic,  255 

simple,  255 

tint,  256-259 

white,  254,  255,  266 
Commutator,  341 
Compass,  293 

used  to  determine  direction  of  elec- 
tric current,  316 

Component  tones  of  notes,  239,  230 
Compound  colors,  254 
Compounds,  159 

classification  of,  205 

formulas  of,  199 

how  named,  207 

inorganic,  175 

organic,  175 

new — ,  formed  by  substitution,  180 
Concave  surface,  51 
Concussion  produces  heat,  126 
Condensation,  change  of  volume,  105 

phase  of  sound  waves,  219 
Conduction,  of  electricity,  305-309, 354 


INDEX 


405 


Conduction,  of  heat,  130,  236 

of  magnetic  force,  299 
Conservation,  of  energy,  386 

of  mass,  183 
Controller,  355 
Convex  surface,  51 
Convection,  cause  of,  136 

of  gases,  135 

of  liquids,  134 
Cooling,  during  evaporation,  123 

during  expansion,  124 
Copper  plate  of  cell,   negative  plate 

with  positive  terminal,  313 
Copper  sulphate  cell,  312 
Core,  soft  iron,  319,  320 
Crooke's  tube,  361,  362 
Crystallization,  94 
Crystals,  alum,  96 

how  produced,  94 

quartz,  96 

salt,  95 

sugar,  96 
Current  of  electricity,  310 

direction  of,  313 

due  to"  chemical  action,  310 

due    to    fluctuation    of    current  in 
neighboring  wire,  345 

due  to  motion  of  wire  across  mag- 
netic field,  335,  336 

fluctuation  in  strength,  336 

induced,  343 

momentary,  335 

primary  and  secondary,  346 
Curvilinear  motion,  381 


DECLINATION,  293 

Decoherer,  360 

Decomposition  of  water  by  electrol- 
ysis, 154 

Deep-sea  fish,  phosphorescent,  19 

Definite  proportions   by  weight,  183, 
186 

Delivery  tube,  143 

Demagnetization,  304 

Density,  of  air,  17-19 
of  ether,  240 


Diaphragm  of  lens  in  camera,  283 
Difference  of  potential,  308-310 
Diffusion  of  liquids,  82 
Direct  current  dynamos,  342 
Direction  of  needle  in  magnetic  field, 

294-297,  314,  315 

Direction  of  strain  in  the  ether,  affect- 
ed by  soft  iron,  299-301 
around  a  wire   carrying  a  current, 

315 

around  a  wire  moving  across  a  mag- 
netic field,  333 

around  a  wire  while  current  in  neigh- 
boring wire  fluctuates,  345 
in  an  electromagnet,  318 
in  field  around  a  magnet,  295-298 
within  a  loop  carrying  a  current,  31 7 
Distance,  action  at  a,  240,  296 
Distance  of  object,  determines  position 

of  image,  280 

determines  size  of  image,  282 
Dogs,  scent  of,  90 
Double  substitution,  180 
Dynamo,  335-337 
Dyne,  381 

EARTH,  attracted  by  sun,  367 

as  a  magnet,  291,  292 

south-seeking  pole  at  north  end  of, 
292 

part  of  circuit  in  telegraphy,  326 
Ebullition,  105 
Electric,  arc,  356 

bell  328 

cell,  311-314 

charge,  305 

circuit,  313 

conduction,  305-309,  354 

current,  307,  310,  332,  345 

light,  355,  356 

potential,  308-311 

waves,  356,  361 

Electricity,  a  phenomenon  of  the  ether, 
364 

current  — ,310 

induced  —  343 

static  — ,  304 


406 


INDEX 


Electrification,  by  contact  and  separa- 
tion, 309,  310 

by  contact  with  electrified  body,  304 

by  chemical  action,  310,  311 

by  induction,  343 

by  motion  of  wire  across  magnetic 
field,  332 

by  fluctuation  of  current  in  neigh- 
boring wire,  345 

by  rubbing,  304,  309 
Electrolysis  of  water,  154 
Electromagnet,  318 

armature  of,  320 

comparative  strength  of,  320,  324 
Electromotive  force,  308 
Electroscope,  305 
Elements,  172 

inert,  177 

in  animal  body,  174 

in  plants,  173 

chief  —  present  in  earth,  173 

usually  found  combined  in  nature, 
177 

symbols  of,  197 
Energy,  385 

conservation  of,  386 

dissipation  of,  388 

formula  for,  386 

how  measured,  385 

kinetic,  385 

potential,  385 
Equations,  203 
Erg,  384 

Ether,  chemical,  evaporation  of,  123 
Ether,  a  theoretical  substance,  240 

transmission  of  heat  and  light,  240- 
243,  278 

magnetic  strain  of,  296,  315,  333,  364 

waves  in,  241,  248,  356,  361 
Evaporation,  87 

cooling  due  to,  123 

effect  of  pressure  on,  1 10 

of  camphor,  87 

of  ice,  88 

of  solids,  87 

of  syrups,  110 

effect  of  on  crystallization,  97-99 


Expansion  due  to  heat,  106 

to  cold  water  on  cooling  below  39.2° 
F.,  126 

water  on  freezing,  105 
Extent  of  atmosphere,  238 
Eye,  structure  of,  284 

FALLING  BODIES,  373,  374 
Field  of  magnet,  294,  296 ;   see  Mag- 
netic field 
Field  magnet,  336 
Film,  soap  bubble,  thickness  of,  80 
Fire-engine,  36 

Fish,  deep-sea,  luminosity  of,  19 
Flow  of  electricity,  307,  311,  333 
Fluctuation  of  strength  of  current  in 

loop  of  dynamo,  338 
Fluoroscope,  362 
Focus  of  lens,  282 
Foot-pound,  384 
Foot-poundal,  384 
Force,  379 

centrifugal,  centripetal,  382,  383 

lines  of  force,  295-298,  315,  333,  344, 
345 

units  of  force,  379 
Force  pump,  36 
Formula,  370 

chemical,  199-203 

for  falling  bodies,  374 
Freezing  mixtures,  123,  124 
Frequency  of  vibration,  215 

of  sound  waves,  216 

of  waves  of  light,  263 
Friction,  heat  produced  by,  127 
Front  of  wave  practically  plane,  247 
Fulcrum,  390 
Fundamental  tones,  229 

GALVANOMETER,  mirror,  332 

sensitive,  331 

tangent,  330 
Gases,  buoyancy  of,  65 

cooling  due  to  expansion  of,  124 

osmosis,  85 

poor  conduction  of  heat,  133 

rate  of  motion  of  molecules  in,  87 


INDEX 


407 


Gases,  relation  between  pressure  and 

volume,  24 
Glass- working,  44,  45 
Gold-leaf,  thickness  of,  80 
Gravitation,  365 

Gravity,  acceleration  due  to,  373 
Gravity  cell,  312 
Ground,  with  ground  plate,   part  of 

electric  circuit,  327 

HEAT,  molecular  motion,  128 
capacity  of  substances  for,  125 
cause  of  —  in  animal  body,  212 
chemical  action  assisted  by,  210 
conduction,  130,  236 
convection,  134,  136 
produced  by  collision,  126 
by  compression,  127 
by  fricton,  127 
by  electric  currents,  355 
by  uniting  of  elements  to   form 

compounds,  211 
quantity  of,  110 
unit  of  quantity  of,  112 
withdrawn  during  evaporation,  123 
during  separation   of  compounds 

into  their  elements,  210 
Heat,  radiant,  235-241 
absorption  of,  276-279 
reflection  of,  268,  276 
transmission  of,  277-279 
variation  in  different  parts  of  spec- 
trum, 260-262 
waves,  invisible,  259-263 

visible,  263 
Horse-power,  384 
Hydraulic  press,  69,  70 
Hydrochloric  acid,  82 
Hydrogen,  2 
flame  of,  147 
production  of,  145 
tests  for,  152 
Hydrogen  dioxide,  187 
Hydrostatic  pressure,  68 

ICE,  evaporation  of,  88 
melting  point  of,  103,  114 


Ice,  temperature  of,  101 
Image,  distance  from  lens,  280 

influenced  by  convexity  of  lens,  283 

inversion  of,  281-283 

size,  282 

Incandescent  lamp,  355 
Inclined  plane,  393,  394 
Induction,  electric,  322,  343 

magnetic,  298 

Induction  coils,  347,  351,  356 
Inertia,  379 

Inorganic  compounds,  175 
Intensity  of  color,  256-259,  286 

of  heat,  260,  261 

of  light,  256-259,  286 

of  sound,  224,  225 

of  vibration,  215,  223 
Internal  'reflection  of  light,  273 
Invisible  spectrum,  261,  263,  208 

waves  of  heat,  261 

KEY,  telegraph,  325 
Kilogram,  380,  399 
Kilogram-metre,  384 
Kinetic  energy,  385 

LATENT  HEAT,  120 

of  water,  121 

of  steam,  121 
Lava,  99 

Lenses,  effect  of  convexity  on  distance 
of  image,  283 

effect  of  margin  on  image  produced 

by  centre  of  lens,  283 
Lever,  390,  391 
Lifting  pump,  33,  34 
Light,  235  ;  see  Color 

absorption  of,  272,  275 

electric,  355,  356 

reflection  of,  265-267,  273 

source  of,  264 
Lightning,  264 
Limit  of  hearing,  216 
Lines  of  force,  295  ;  see  Direction  of 
strain  in  the  ether 

complete  circuits,  297.  315,  333,  345 

number  of,  319,  336,  345 


408 


INDEX 


Liquefaction  change  of  volume  during, 

104 

Liquid  air,  139 
Liquid  films,  tension  of,  41 
Liquids,   contraction  due    to  loss  of 
heat,  76 

diffusion  of,  82 

osmosis  of,  83 

poor  conductors  of  heat,  133 

pressure  downward,  57,  59 

upward,  61 

specific  gravity,  75 
Litmus,  effect  of  acids  on,  81 

effect  of  alkalis  on,  82 
Loop,  bearing  current,  acts  like  a  mag- 
net, 317 

rotated  in  a  magnetic  field  carries  a 

current,  335,  338 
Loops  and  nodes  of  strings,  228 
Loudness  of  sound,  219,  223 

MACHINES,  efficiency  of,  389 

general  law  of,  389 

simple,  390 

useful  work  of,  389 
Magnet,  artificial,  288 

earth  a  magnet,  291 

electromagnet,  318 

field  magnet,  294-297 

loop  bearing  a  current  acts  like  a 

magnet,  317 
Magnetic  circuit,  297,  315,  333 

conductivity  of,  299,  319 

declination  of,  293 

field,  294,  296,  315,  333,  344,  364 

induction,  298 

influence  on  direction  of  needle,  294— 
297,  315,  330 

lines  of  force,  295-298,  815 

needle  of  compass,  293 

permeability,  300,  302 

poles,  290,  291 
of  earth,  292 

substances,  290 

transparency,  290 
Magnetism,  289 

ether  theory  of,  296  315 


Magnetite,  288 
Magnetization,  302-304 
Manometer,  39 
Mass,  369 

distinguished  from  weight,  380 
Matter,  states  of,  100 
Mechanics,  365 
Medium,  of  electric  waves,  356,  361 

of  magnetic  force,  296,  315,  333,  364 

of  radiant  heat  and  light,  240-243, 
278 

of  sound,  217 
Melting  point,  of  ice,  103 

effect  of  pressure  on,  107 
Membrane,  diffusion  through,  83-85 

drum  of  ear,  231 
Mercury,  barometer,  15 

cohesion  compared  with  adhesion  to 
glass,  51 

convex  surface  in  glass  tubes,  51 ,  52 

depressed  in  capillary  tubes,  50 

drop  of,  why  spherical,  43 

mirror,  160 

properties  of,  160 

red  oxide  of,  160 
Metals,  198,  205 

conductors  of  electricity,  354 

of  heat,  130 
Meteorites,  238 
Metre,  399 
Metric  tables,  399 
Microscope,  size  of  smallest  particles 

detected  by,  80,  91 
Minerals,  173 
Mixtures,  159 
Molecular  theory,  77 

of  heat,  128 

of  magnetism,  302 

Molecules,    driven    farther    apart    by 
heat,  77,  78 

motions  of,  in  heat,  128 
in  transmitting  sound,  221,  222 

of  compounds,  192 

relative  size,  81 

small  size  indicated  by  gold-leaf  and 
by  soap-bubble  film,  80 ;  by  sense 
of  smell,  89 


INDEX 


409 


Momentum,  368 
Monochord,  228 
Monochromatic  light,  251,  253 
Moon;  attracted  by  earth,  367 
Morse  code,  322 
Motion,  curvilinear,  381 

quantity  of,  368 

uniform,  370 

uniformly  accelerated,  371 
•    formula  for,  373 
Motors,  342 
Multiple  proportion   by   weight,  186- 

189 

Musical  pitch,  216 
Musk,  89 

NEGATIVE,  pole  of  magnet  is  south- 
seeking,  290 

pole  of  earth  is  at  north  end,  292 

terminal  of  electric  cell  is  at  top  of 

zinc  plate,  313 
Nerves  of  hearing,  234 

of  pain,  137 

of  temperature,  137 

of  touch,  137 

of  sight,  263,  286 
Nitric  acid,  82 
Nitrogen,  amount  in  air,  176 

dioxide,  188 

how  secured,  150 

monoxide,  188 

pentoxide,  188 

present  in  air,  151 

properties  of,  152 

tests  for,  152 

tetroxide,  188 

trioxide,  188 

Nodes  of  vibrating  strings,  228 
Non-conductors,    of    electricity,    309, 
325 

of  heat,  131 
Non-metals,  198,  205 

OPAQUE  OBJECTS,  color  of,  265,  273 
Ores,  193 

Organic  compounds,  175 
Oscillating  discharges,  357 


Osmose,  of  gases,  85 

of  liquids,  83 
Overflow  can,  70 
Overtones,  229 
Oxidation,  212 
Oxide,  definition  of,  205 
Oxygen,  amount  in  air,  144 

amount  in  human  body,  174 

production  of,  142 

properties  of,  143 

tests  for,  152 

PAIN,  sensation  of,  137 
Parallel  connection  of  cells,  313 
Pascal's  Law,  67 
Pendulum,  374 

amplitude  of  vibration,  376 

centre  of  oscillation,  377 
of  suspension,  375 

compound,  377 

law  of,  377 

length  of  compound  pendulum,  377 
of  seconds-pendulum,  377 

motion  described,  375 

period  of  vibration,  37(5 

rate  of  vibration,  370 

reversible,  378 

simple,  375 

uses  of,  378 

Perception  of  color,  263,  285 
Period  of  vibration,  215,  376 
Permanent  magnet,  289,  349 
Permeability,  magnetic,  300,  302,  319. 

336 

Phenomenon  denned,  48 
Phosphorus,  how  stored,  177 
Pigments,  color  of,  274 
Piston  25 
Pitch,  215 

in  musical  instruments,  216 

range  in  human  voice,  217 

varies  with  length  in  strings,  227 
Pitch  of  screw,  396 
Plane,  inclined,  393*,  394 
Plane  wave  front,  247 
Platinum  wire,  how  fastened  in  glass 
tube,  46 


410 


INDEX 


Pneumatic  trough,  140 

Poles  of  earth,  geographic,  292 

difference  in  action  of,  291 

magnetic,  292 

of  electromagnets,  319 

of  magnets,  290 
Position  of  image,  280-283 
Positive  pole  of  earth  is  in  Antarctic 
regions,  292 

of  magnets,  north-seeking,  290 

terminals  of   cells  at  top  of  carbon 

or  copper  plates,  313 
Potassium  bichromate  cell,  313,  314 
Potential,  310,  311 
Potential  energy,  385 
Pound,  380 
Poundal,  381 
Power,  384 
Precipitates,  167 

separation  of,  170 
Pressure,  effect  on  boiling  point,  108 

effect  on  melting  point,  107 

hydrostatic,  67,  68 

relation  to  volume  in  gases,  24 
Primary  circuit  or  current,  346 
Primary  colors,  255 
Prism,  refraction  of  light  by,  250 

of  rock  salt,  276 
Prismatic  colors,  255,  257 
Propagation  of  waves  of  sound;  see  Air 

of  radiant  heat,  light,  and  electric- 
ity;  see  Ether 
Pulley,  392 
Pump-barrel,  32 
Pump-plate,  28 
Pumps,  25-38 

valves  of,  25 

QUALITY  OF  SOUND,  229 
Quantity  of  heat,  110,  111 

unit  of,  111 

Quantity  of  matter  ;  see  Mass 
Quantity  of  motion ;  see  Momentum 
Quartz,  crystals,  96 

RADIANT  ENERGY,  235 
Radiant  heat,  235 


Radiation  of  heat,  241-243 

in  relation  to  absorption,  279 
Rain  foretold  by  barometer,  16 
Rarefaction  and  condensation  in  waves 

of  air,  219 
Rays  of  light,  243 

Roentgen,  360 
Reagents,  168 
Receiver  of  air-pump,  29 

of  telephone,  349 
Red  light,  refraction  of,  253 
Reflection,  of  heat,  268 

in  relation  to  absorption,  276 

internal,  273 

of  light,  265-267,  270,  276 
Refraction,  of  light,  243 

cause  of,  248 

from  denser  to  rarer  medium,  245 

from  rarer  to  denser,  246 

of  red  light,  253 

of  yellow  light,  251 

through  a  prism,  250 
Relay,  323 
Repulsion,  of  like  electric  charges,  305 

of  like  poles,  291 
Resistance,  355 
Rings,  conducting,  341 
Rock  salt  prism,  276 
Roentgen  rays,  360 
Rose,  cause  of  odor,  88 
Rose-oil,  89 
Ruhmkorff  coils,  347,  356 

SAL  AMMONIAC  CELL,  312 
Salt,  crystals  of,  95 
Salts,  how  formed,  205,  206 
Saturated  solution,  93 
Screen,  magnetic,  301 
Screw,  395 

law,  396 

pitch,  396 

Sea-level,  air  pressure  at,  14 
Secondary  circuit  or  current,  346 
Segments  of  vibrating  strings.  228 
Self-luminous  bodies,  264 
Sensation,  of  hearing,  218:  233 

of  pain,  137 


INDEX 


411 


Sensation  of  sight,  285 

of  taste,  92 

of  temperature,  137 

of  touch,  137 

Sensitive  galvanometer,  331 
Sensitiveness  of  eye  to  different  colors, 

286 

Series  connection  of  cells,  326,  327 
Shooting  stars,  238 
Simple  machines,  390 
Simple  pendulum,  375 
Siphon,  54-56 

principle  of,  55 

use  of,  56 
Smell,  cause  of,  88 
Soap-bubble,  thickness  of  film,  80 
Sodium,  color  of  flame,  251 

how  stored,  177 
Soft  iron,  armature,  320,  336 

influence  on  direction  of  strain  in 

a  magnetic  field,  299,  300,  319 
Solar  spectrum,  255 

relative  heat  in   different  parts  of, 

260 

Solidification,  change  of  volume  dur- 
ing, 104 
Solids,  relative  conductivity  of  heat, 

130 
Solution,  cause,  90 

cold  produced  by,  123 

saturated,  93 

unsaturated,  93 
Solutions,  effect  of  heat  on,  92 

effect  of  evaporation,  93 

effect  of  cooling  on,  93 

small  size  of  molecules  indicated  by, 

91 
Sound,  214 

due  to  vibration  of  sounding  bodies, 
214 

intensity  and  loudness,  223 

pitch,  216,  227 

quality  due  to  overtones,  230 

sensation,  218,  233 

transmitted  by  vibration  of  mole- 
cules in  air,  222 

transmitted  by  waves  in  air,  219 


Sound  velocity,  230 
Sounder,  321 
Spark,  electric,  357 
Specific  density,  73 
Specific  gravity,  defined,  71 

of  iron,  72 

of  liquids,  75 

of  solids  heavier  than  water,  72 

of  solids  lighter  than  water,  73 
Spectrum,  growth  during  rise  of  tem- 
perature, 255 

invisible,  268 

solar,  255 

ultra -red  and  ultra-violet,  263 

visible  part  of,  263,  268 
States  of  matter,  1 00 
Static  electricity,  304 
Steam,  dry,  103 

latent  heat  of,  120,  122 

temperature,  103 
Stop-cocks,  38 
Straight  path  of  light,  243 
Strain  of  ether,   direction  of,  296,  297, 
315 

influenced  by  soft  iron,  299,  319,  336, 

344 
Strings,  vibration  of,  227 

vibration  in  parts,  228 
Sublimation,  87 
Substitution,  chemical,  165 

new  compounds  formed  by,  180 
Sugar,  crystals  of,  96 

granulated,  99 
Sulphuric  acid,  82 
Sun,  attraction  for  earth,  367 
Sun's  light,  color  is  white,  254 

transmitted  by  ether,  240 
Surface  tension  of  liquids,  42 
Symbols  of  elements,  197 
Sympathetic  vibrations,  226,  234 
Synthesis,  chemical,  160 

TANGENT  GALVANOMETER,  330 

Taste,  92 

Telegraph,  code  or  alphabet,  322 

key,  325 

relay,  323 


412 


INDEX 


Telegraph  sounder,  821 

wireless,  358-360 
Telephone,  347-354 

receiver,  349 

transmitter,  348 

use  of  induction  coil,  351 
Temperature,  defined,  110 

decrease  with  elevation,  237 

extremes  of,  139 

measurement  of,  110 

of  ice,  101 

of  steam,  103 

of  water,  102 

sensation  of,  13  < 

unit  of,  110 
Tension,  of  strings,  227 

of  surface  film  of  liquids,  42 
Terminals  of  cells,  313 
Terrestrial  magnetism,  292 
Theory,  atomic,  193,  194 

ether,   theory  of  radiant  heat  and 
light,  240 

of  electricity,  358,  362 

of  magnetism,  295,  315,  362 

lines  of  magnetic  force,  295 

molecular,  77 

molecular  theory  of  heat,  128 

molecular  theory  of  magnetism,  302 
Thermopile,  260 
Thistle  tube,  82 
Thumb  and  finger  rule,  316,  317,  319, 

329,  334 

Tint  of  color,  259 

Tones,  frequency  of  vibration  or  pitch, 
216 

fundamental,  229 

overtones,  229 

Translation,  motion  of  molecules,  128 
Translucency,  272 

Transmission    of    heat,    by     contact, 
236 

by  radiation,  241-243 

in  relation  to  absorption,  277 
Transmission  of  light,  268-275 

selective  transmission  of  visible  and 

invisible  waves,  275, 276 
Transmission  of  sound,  217 


Transmission  of  many  sound  waves 
simultaneously,  230 

by  liquid  in  labyrinth,  234 
Transmitter,  348 

Transparency  of  thin  sections,  272 
Transparent  objects,  colorless,  268 

color  due  to  selective  absorption,  269 
Tubes,  capillary,  49,  50 

ULTRA-RED  AND  ULTRA-VIOLET  SPEC- 
TRUM, 263 
U-tubes,  balancing  of  liquids  in,  20 

VACUUM,  falling  bodies  in  a,  373 
transmission  through,  of  electricity, 

361 

of  electrical  discharges,  360 
of  heat,  239 
of  light,  239 
of  magnetic  force,  296 
Vacuum  fountain,  7 
Vacuum  pans,  110 
Valves,  24 

Variation,  magnetic,  293 
Velocity,  368 
accelerated,  371,  372 
average,  372 
of  electricity,  364 
of  molecules  in  gases,  87 
of  molecules  in  liquids,  83 
of  sound,  230 
uniform,  370,  371 
uniformly  accelerated,  372 
Vibrations,  375 
amplitude  of,  215,  375,  376 
frequency  of,  215,  223 
intensity  of,  215,  223 
period  of,  215,  376 
of  molecules  in  heat,  128,  129 
of  molecules  in  sound,  222 
of  sounding  bodies,  214 
of  strings,  228 

of  atoms  in  radiant  heat  and  light,  242 
rate  of,  376 

Visibility,  depends  on  light  received, 
and  then  reflected  or  trans- 
mitted, 265 


INDEX 


413 


Voice,  217 

Volume,   change  during  liquefaction, 

or  consolidation,  104 
during  vaporization,  105,  106 

WATER,  attraction  for  glass,  47 

attraction  for  water,  cohesion,  46 

boiling  point  of,  104 

capillary  action  of,  48 

chemical  action  assisted  by,  209 

composition  of,  154 

cohesion  compared  with  adhesion  to 
glass,  51 

drop  of,  why  spherical,  43 

electrolysis,  154 

expansion  due  to  freezing,  105 

freezing  point  of,  104 

irregular  expansion  of,  125 

maximum  density  of,  71 

poor  conductor  of  heat,  133 

pressure  downward,  59,  60 

pressure  upward,  61 

production  of.  157 

relative  proportion  of  oxygen  and  hy- 
drogen, 156 

standard  for  specific  gravity,  71 

weight  of  one  cubic  foot  of,  72 

weight  of  one  cubic  inch  of,  63 
Water  pump,  action  like  air  pump,  32 
Wave  front  practically  flat,  247 


Waves,  of  condensation  and   rarefac- 
tion, 219 
electric,  356,  361 
in  ether,  241,  243 
motion   of  individual    molecules  in 

air,  219 

of  light,  chemical  effects,  279 
simultaneous  transmission  of  many 

waves  in  air,  230 
Wedge,  394,  395 
Weight,  atomic,  196 
distinguished  from  mass,  380 
loss  due  to  immersion,  of  body  in  a 

liquid,  63 
variation  with  latitude  and  altitude, 

380 

Werkojank,  102 
Wheel  and  axle,  391 
Wheel,  Savart's,  215 
White  light,  254,  255,  266 
Wireless  telegraphy,  358,  359 
Work,  384 

X-RATS,  362 

YELLOW    LIGHT,   refraction  of,   251, 
253 

ZERO,  absolute,  139 
Zinc  plate  of  cell,  positive  plate  with 
negative  terminal,  313 


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